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History of smallpox
History of smallpox
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The history of smallpox extends into pre-history.[1] Genetic evidence suggests that the smallpox virus emerged 3,000 to 4,000 years ago.[2] Prior to that, similar ancestral viruses circulated, but possibly only in other mammals, and possibly with different symptoms. Only a few written reports dating from about 500–1000 CE are considered reliable historical descriptions of smallpox, so understanding of the disease prior to that has relied on genetics and archaeology. However, during the second millennium, especially starting in the 16th century, reliable written reports become more common.[2] The earliest physical evidence of smallpox is found in the Egyptian mummies of people who died some 3,000 years ago.[3] Smallpox has had a major impact on world history, not least because indigenous populations of regions where smallpox was non-native, such as the Americas and Australia, were rapidly and greatly reduced by smallpox (along with other introduced diseases) during periods of initial foreign contact, which helped pave the way for conquest and colonization. During the 18th century, the disease killed an estimated 400,000 Europeans each year, including five reigning monarchs, and was responsible for a third of all blindness.[4] Between 20 and 60% of all those infected—and over 80% of infected children—died from the disease.[5]

During the 20th century, it is estimated that smallpox was responsible for 250–500 million deaths.[6][7][8] In the early 1950s, an estimated 50 million cases of smallpox occurred in the world each year.[9] As recently as 1967, the World Health Organization estimated that 15 million people contracted the disease and that two million died in that year.[9] After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the global eradication of smallpox in May 1980.[9] Smallpox is one of two infectious diseases to have been eradicated, the other being rinderpest, which was declared eradicated in 2011.[10][11][12]

Eurasian epidemics

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It has been suggested that smallpox was a major component of the Plague of Athens that occurred in 430 BCE, during the Peloponnesian Wars, and was described by Thucydides.

Galen's description of the Antonine Plague, which swept through the Roman Empire in 165–180 CE, indicates that it was probably caused by smallpox.[13] Returning soldiers picked up the disease in Seleucia (in modern Iraq), and brought it home with them to Syria and Italy. It raged for fifteen years and greatly weakened the Roman empire, killing up to one-third of the population in some areas.[14] Total deaths have been estimated at 5 million.[15]

A second major outbreak of disease in the Roman Empire, known as the Plague of Cyprian (251–266 CE), was also either smallpox or measles. The Roman empire stopped growing as a consequence of these two plagues, according to historians such as Theodore Mommsen. Although some historians believe that many historical epidemics and pandemics were early outbreaks of smallpox, contemporary records are not detailed enough to make a definite diagnosis.[1][16] The discovery of smallpox-related osteomyelitis on a skeleton buried at Corinium in the late 3rd century confirms the presence of the disease in the Roman world around this time, though not its ubiquity.[17]

Written sometime before 400 AD, the Indian medical book Sushruta Samhita recorded a disease marked by pustules and boils, saying "the pustules are red, yellow, and white and they are accompanied by burning pain … the skin seems studded with grains of rice."[18] The Indian epidemic was thought to be punishment from a god, and the survivors created a goddess, Sitala, as the anthropomorphic personification of the disease.[19][20][21] Smallpox was thus regarded as possession by Sitala. In Hinduism the goddess Sitala both causes and cures high fever, rashes, hot flashes and pustules. All of these are symptoms of smallpox.[citation needed]

Most of the details about the epidemics are lost, probably due to the scarcity of surviving written records from the Early Middle Ages. The first incontrovertible description of smallpox in Western Europe occurred in 581 AD, when Bishop Gregory of Tours provided an eyewitness account describing the characteristic symptoms of smallpox.[16] Waves of epidemics wiped out large rural populations.[22]

In 710 AD, smallpox was re-introduced into Europe via Iberia by the Umayyad conquest of Hispania.[23]

The Japanese smallpox epidemic of 735–737 is believed to have killed as much as one-third of Japan's population.[24][25]

There is evidence that smallpox reached the Philippines from the 4th century onwards possibly from indirect trade with Indians.[26]

During the 18th century, there were many major outbreaks of smallpox, driven possibly by increasing contact with European colonists and traders. There were epidemics, for instance, in the Sultanate of Banjar (South Kalimantan), in 1734, 1750–51, 1764–65 and 1778–79; in the Sultanate of Tidore (Moluccas ) during the 1720s, and in southern Sumatra during the 1750s, the 1770s and in 1786.[26][27][28]

The clearest description of smallpox from pre-modern times was given in the 9th century by the Persian physician, Muhammad ibn Zakariya ar-Razi, known in the West as "Rhazes", who was the first to differentiate smallpox from measles and chickenpox in his Kitab fi al-jadari wa-al-hasbah (The Book of Smallpox and Measles).[29]

Female smallpox patient in London, c. 1890

Smallpox was a leading cause of death in the 18th century. Every seventh child born in Russia died from smallpox.[9] It killed an estimated 400,000 Europeans each year in the 18th century, including five reigning European monarchs.[30] Most people became infected during their lifetimes, and about 30% of people infected with smallpox died from the disease, presenting a severe selection pressure on the resistant survivors.[31]

In northern Japan, Ainu population decreased drastically in the 19th century, due in large part to infectious diseases like smallpox brought by Japanese settlers pouring into Hokkaido.[32]

The Franco-Prussian War triggered a smallpox pandemic of 1870–1875 that claimed 500,000 lives; while vaccination was mandatory in the Prussian army, many French soldiers were not vaccinated. Smallpox outbreaks among French prisoners of war spread to the German civilian population and other parts of Europe. Ultimately, this public health disaster inspired stricter legislation in Germany and England, though not in France.[33]

In 1849 nearly 13% of all Calcutta deaths were due to smallpox.[34] Between 1868 and 1907, there were approximately 4.7 million deaths from smallpox in India. Between 1926 and 1930, there were 979,738 cases of smallpox with a mortality of 42.3%.[35]

African epidemics

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Variola lesions on chest and arms

One of the oldest records of what may have been an encounter with smallpox in Africa is associated with the elephant war circa AD 568 CE, when after fighting a siege in Mecca, Ethiopian troops contracted the disease which they carried with them back to Africa.[citation needed]

Arab ports in Coastal towns in Africa likely contributed to the importation of smallpox into Africa, as early as the 13th century, though no records exist until the 16th century. Upon invasion of these towns by tribes in the interior of Africa, a severe epidemic affected all African inhabitants while sparing the Portuguese. Densely populated areas of Africa connected to the Mediterranean, Nubia and Ethiopia by caravan route likely were affected by smallpox since the 11th century, though written records do not appear until the introduction of the slave trade in the 16th century.[36]

The enslavement of Africans continued to spread smallpox to the entire continent, with raiders pushing farther inland along caravan routes in search of people to enslave. The effects of smallpox could be seen along caravan routes, and those who were not affected along the routes were still likely to become infected either waiting to be put onboard or on board ships.[36]

Smallpox in Angola was likely introduced shortly after Portuguese settlement of the area in 1484. The 1864 epidemic killed 25,000 inhabitants, one third of the total population in that same area. In 1713, an outbreak occurred in South Africa after a ship from India docked at Cape Town, bringing infected laundry ashore. Many of the settler European population suffered, and whole clans of the Khoisan people were wiped out. A second outbreak occurred in 1755, again affecting both the white population and the Khoisan. The disease spread further, completely eradicating several Khosian clans, all the way to the Kalahari desert. A third outbreak in 1767 similarly affected the Khoisan and Bantu peoples. But the European colonial settlers were not affected nearly to the extent that they were in the first two outbreaks, it has been speculated this is because of variolation. Continued enslavement operations brought smallpox to Cape Town again in 1840, taking the lives of 2500 people, and then to Uganda in the 1840s. It is estimated that up to eighty percent of the Griqua tribe was exterminated by smallpox in 1831, and whole tribes were being wiped out in Kenya up until 1899. Along the Zaire river basin were areas where no one survived the epidemics, leaving the land devoid of human life. In Ethiopia and the Sudan, six epidemics are recorded for the 19th century: 1811–1813, 1838–1839, 1865–1866, 1878–1879, 1885–1887, and 1889–1890.[36]

Epidemics in the Americas

[edit]
Documented smallpox epidemics in the New World[37]
Year Location Description
1520–1527 Mexico, Central America, South America Smallpox kills 5–8 millions of native inhabitants of Mexico. Unintentionally introduced at Veracruz with the arrival of Panfilo de Narvaez on April 23, 1520, and was credited with the victory of Cortes over the Aztec empire at Tenochtitlan (present-day Mexico City) in 1521. Kills the Inca ruler, Huayna Capac, and 200,000 others and weakens the Incan Empire.
1561–1562 Chile No precise numbers on deaths exist in contemporary records but it is estimated that natives lost 20 to 25 percent of their population. According to Alonso de Góngora Marmolejo, so many Indian laborers died that the Spanish gold mines had to shut down.[38]
1588–1591 Central Chile A combined smallpox, measles and typhus plague strikes Central Chile contributing to a decline of indigenous populations.[39]
1617–1619 North America northern east coast Killed 90% of the Massachusetts Bay Indians
1655 Chillán, Central Chile An outbreak of smallpox occurred among refugees from Chillán as the city was evacuated amidst the Mapuche uprising of 1655. Spanish authorities put this group in effective quarantine decreeing death sentences for anyone crossing Maule River north.[40]
1674 Cherokee Tribe Death count unknown. Population in 1674 about 50,000. After 1729, 1738, and 1753 smallpox epidemics their population was only 25,000 when they were forced to Oklahoma on the Trail Of Tears.
1692 Boston, MA
1702–1703 St. Lawrence Valley, NY
1721 Boston, MA A British sailor disembarking HMS Seahorse brought smallpox to Boston. 5759 people were infected and 844 died.
1736 Pennsylvania
1738 South Carolina
1770s West Coast of North America 1770s Pacific Northwest smallpox epidemic At least 30% (tens of thousands) of the Northwestern Native Americans die from smallpox.[41][42]
1781–1783 Great Lakes
1830s Alaska Reduced Dena'ina Athabaskan population in Cook Inlet region of southcentral Alaska by half.[43] Smallpox also devastated Yup'ik Eskimo populations in western Alaska.
1836–1840 Great Plains 1837 Great Plains smallpox epidemic
1860–1861 Pennsylvania
1862 British Columbia, Washington state & Russian America Known as the Great Smallpox of 1862, an outbreak of smallpox in a large encampment of all indigenous peoples from around the colony on June 10, 1862, dispersed by order of the government to return to their homes, resulted in the deaths of 50–90% of the indigenous peoples in the region[44][45][46][47][48]
1865–1873 Philadelphia, PA, New York, Boston, MA and New Orleans, LA Same period of time, in Washington D.C., Baltimore, MD, Memphis, TN, Cholera and a series of recurring epidemics of Typhus, Scarlet Fever and Yellow Fever
1869 Araucanía, southern Chile A smallpox epidemic breaks out among native Mapuches, just some months after a destructive Chilean military campaign in Araucanía.[49]
1877 Los Angeles, CA
1880 Tacna, Peru Tacna hosted the combined armies of Peru and Bolivia before being defeated by Chile in the Battle of Tacna. Before it fell to Chileans in late May 1880 infectious diseases were widespread in the city with 461 deaths of smallpox in the 1879–1880 period, making up 11.3% of all registered deaths for the city in the same period.[50]
1885 Montréal, Québec

3164 dead in Montréal (municipality), 5864 for the whole province of Québec. (fr) Smallpox in Montréal in 1885

1902 Boston, Massachusetts Of the 1,596 cases reported in this epidemic, 270 died.
1905 Southern Patagonia, Chile A smallpox epidemic hits Tehuelche communities in Magallanes Territory, Chile.[51][52] Cacique José Mulato died in the epidemic.[52]

After first contacts with Europeans and Africans, some believe that the death of 90–95% of the native population of the New World was caused by Old World diseases.[53] It is suspected that smallpox was the chief culprit and responsible for killing nearly all of the native inhabitants of the Americas. For more than 200 years, this disease affected all new world populations, mostly without intentional European transmission, from contact in the early 16th century until possibly as late as the French and Indian Wars (1754–1767).[54]

In 1519 Hernán Cortés landed on the shores of what is now Mexico and what was then the Aztec Empire. In 1520 another group of Spanish arrived in Mexico from Hispaniola, bringing with them the smallpox which had already been ravaging that island for two years. When Cortés heard about the other group, he went and defeated them. In this contact, one of Cortés's men contracted the disease. When Cortés returned to Tenochtitlan, he brought the disease with him.[citation needed]

Soon, the Aztecs rose up in rebellion against Cortés and his men. Outnumbered, the Spanish were forced to flee. In the fighting, the Spanish soldier carrying smallpox died. Cortés would not return to the capital until August 1521. In the meantime smallpox devastated the Aztec population. It killed most of the Aztec army and 25% of the overall population.[55] The Spanish Franciscan Motolinia left this description: "As the Indians did not know the remedy of the disease…they died in heaps, like bedbugs. In many places it happened that everyone in a house died and, as it was impossible to bury the great number of dead, they pulled down the houses over them so that their homes become their tombs."[56] On Cortés's return, he found the Aztec army's chain of command in ruins. The soldiers who still lived were weak from the disease. Cortés then easily defeated the Aztecs and entered Tenochtitlán.[57] The Spaniards said that they could not walk through the streets without stepping on the bodies of smallpox victims.[58]

The effects of smallpox on Tahuantinsuyu (or the Inca empire) were even more devastating. Beginning in Colombia, smallpox spread rapidly before the Spanish invaders first arrived in the empire. The spread was probably aided by the efficient Inca road system. Within months, the disease had killed the Incan Emperor Huayna Capac, his successor, and most of the other leaders. Two of his surviving sons warred for power and, after a bloody and costly war, Atahualpa become the new emperor. As Atahualpa was returning to the capital Cuzco, Francisco Pizarro arrived and through a series of deceits captured the young leader and his best general. Within a few years smallpox claimed between 60% and 90% of the Inca population,[59] with other waves of European disease weakening them further. A handful of historians argue that a disease called Bartonellosis might have been responsible for some outbreaks of illness, but this opinion is in the scholarly minority.[60] The effects of Bartonellosis were depicted in the ceramics of the Moche people of ancient Peru.[61]

Even after the two largest empires of the Americas were defeated by the virus and disease, smallpox continued its march of death. In 1561, smallpox reached Chile by sea, when a ship carrying the new governor Francisco de Villagra landed at La Serena. Chile had previously been isolated by the Atacama Desert and Andes Mountains from Peru, but at the end of 1561 and in early 1562, it ravaged the Chilean native population. Chronicles and records of the time left no accurate data on mortality but more recent estimates are that the natives lost 20 to 25 percent of their population. The Spanish historian Marmolejo said that gold mines had to shut down when all their Indian labor died.[62] Mapuche fighting Spain in Araucanía regarded the epidemic as a magical attempt by Francisco de Villagra to exterminate them because he could not defeat them in the Arauco War.[38]

In 1633 in Plymouth, Massachusetts, the Native Americans were struck by the virus. As it had done elsewhere, the virus wiped out entire population groups of Native Americans. It reached Mohawks in 1634,[63] the Lake Ontario area in 1636, and the lands of the Iroquois by 1679.[64]

A particularly virulent sequence of smallpox outbreaks took place in Boston, Massachusetts. From 1636 to 1698, Boston endured six epidemics. In 1721, the most severe epidemic occurred. The entire population fled the city, bringing the virus to the rest of the Thirteen Colonies.[65][66]

During the siege of Fort Pitt, as recorded in his journal by sundries trader and militia Captain, William Trent, on June 24, 1763, dignitaries from the Delaware tribe met with Fort Pitt officials, warned them of "great numbers of Indians" coming to attack the fort, and pleaded with them to leave the fort while there was still time. The commander of the fort refused to abandon the fort. Instead, the British gave as gifts two blankets, one silk handkerchief and one linen from the smallpox hospital, to two Delaware Indian delegates.[67][68] The dignitaries were met again later and they seemingly hadn't contracted smallpox.[69] A relatively small outbreak of smallpox had begun spreading earlier that spring, with a hundred dying from it among Native American tribes in the Ohio Valley and Great Lakes area through 1763 and 1764.[69] The effectiveness of the biological warfare itself remains unknown, and the method used is inefficient compared to respiratory transmission and these attempts to spread the disease are difficult to differentiate from epidemics occurring from previous contacts with colonists,[70] as smallpox outbreaks happened every dozen or so years.[71]

In the late 1770s, during the American Revolutionary War, smallpox returned once more and killed thousands.[72][73][74] Peter Kalm in his Travels in North America, described how in that period, the dying Indian villages became overrun with wolves feasting on the corpses and weakened survivors.[75] During the 1770s, smallpox killed at least 30% of the Northwestern Native Americans, killing tens of thousands.[41][42] The smallpox epidemic of 1780–1782 brought devastation and drastic depopulation among the Plains Indians.[76] This epidemic is a classic instance of European immunity and non-European vulnerability. It is probable that the Indians contracted the disease from the 'Snake Indians' on the Mississippi. From there it spread eastward and northward to the Saskatchewan River. According to David Thompson's account, the first to hear of the disease were fur traders from the Hudson's House on October 15, 1781.[77] A week later, reports were made to William Walker and William Tomison, who were in charge of the Hudson and Cumberland Hudson's Bay Company posts. By February, the disease spread as far as the Basquia Tribe. Smallpox attacked whole tribes and left few survivors. E. E. Rich described the epidemic by saying that "Families lay unburied in their tents while the few survivors fled, to spread the disease."[78] After reading Tomison's journals, Houston and Houston calculated that, of the Indians who traded at the Hudson and Cumberland houses, 95% died of smallpox.[76] Paul Hackett adds to the mortality numbers suggesting that perhaps up to one-half to three-quarters of the Ojibway situated west of the Grand Portage died from the disease. The Cree also suffered a casualty rate of approximately 75% with similar effects found in the Lowland Cree.[79] By 1785 the Sioux Indians of the great plains had also been affected.[36] Not only did smallpox devastate the Indian population, it did so in an unforgiving way. William Walker described the epidemic stating that "the Indians [are] all Dying by this Distemper … lying Dead about the Barren Ground like a rotten sheep, their Tents left standing & the Wild beast Devouring them."[77]

In 1799, the physician Valentine Seaman administered the first smallpox vaccine in the United States. He gave his children a smallpox vaccination using a serum acquired from Edward Jenner, the British physician who invented the vaccine from fluid taken from cowpox lesions. Though vaccines were misunderstood and mistrusted at the time, Seaman advocated their use and, in 1802, coordinated a free vaccination program for the poor in New York City.[80][81]

By 1832, the federal government of the United States established a smallpox vaccination program for Native Americans.[82]

In 1900 starting in New York City, smallpox reared its head once again and started a sociopolitical battle with lines drawn between the rich and poor, white and black. In populations of railroad and migrant workers who traveled from city to city the disease had reached an endemic low boil. This fact did not bother the government at the time, nor did it spur them to action. Despite the general acceptance of the germ theory of disease, pioneered by John Snow in 1849, smallpox was still thought to be mostly a malady that followed the less-distinct guidelines of a "filth" disease, and therefore would only affect the "lower classes".[83]

The last major smallpox epidemic in the United States occurred in Boston, Massachusetts throughout a three-year period, between 1901 and 1903. During this three-year period, 1596 cases of the disease occurred throughout the city. Of those cases, nearly 300 people died. As a whole, the epidemic had a 17% fatality rate.[84]

Indigenous victims (likely smallpox), Florentine Codex (compiled 1540–1585)

Those who were infected with the disease were detained in quarantine facilities in the hopes of protecting others from getting sick. These quarantine facilities, or pesthouses, were mostly located on Southampton Street. As the outbreak worsened, men were also moved to hospitals on Gallop's Island. Women and children were primarily sent to Southampton Street. Smallpox patients were not allowed in regular hospital facilities throughout the city, for fear the sickness would spread among the already sick.[85]

A reflection of the previous outbreak that occurred in New York, the poor and homeless were blamed for the sickness's spread. In response to this belief, the city instructed teams of physicians to vaccinate anyone living in inexpensive housing.[citation needed]

In an effort to control the outbreak, the Boston Board of Health began voluntary vaccination programs. Individuals could receive free vaccines at their workplaces or at different stations set up throughout the city. By the end of 1901, some 40,000 of the city's residents had received a smallpox vaccine. However, despite the city's efforts, the epidemic continued to grow. In January 1902, a door-to-door vaccination program was initiated. Health officials were instructed to compel individuals to receive vaccination, pay a $5 fine, or face 15 days in prison. This door-to-door program was met by some resistance as some individuals feared the vaccines to be unsafe and ineffective. Others felt compulsory vaccination in itself was a problem that violated an individual's civil liberties.[citation needed]

This program of compulsory vaccination eventually led to the famous Jacobson v. Massachusetts case. The case was the result of a Cambridge resident's refusal to be vaccinated. Henning Jacobson, a Swedish immigrant, refused vaccination out of fear it would cause him illness. He claimed a previous smallpox vaccine had made him sick as a child. Rather than pay the five dollar fine, he challenged the state's authority on forcing people to receive vaccination. His case was lost at the state level, but Jacobson appealed the ruling, and so, the case was taken up by the Supreme Court. In 1905 the Supreme Court upheld the Massachusetts law: it was ruled Jacobson could not refuse the mandatory vaccination.[84]

In Canada, between 1702 and 1703, nearly a quarter of the population of Quebec city died due to a smallpox epidemic.[36]

Pacific epidemics

[edit]

Australia

[edit]

Smallpox was brought to Australia in the 18th century. The first recorded outbreak, in April 1789, about 16 months after the arrival of the First Fleet, devastated the Aboriginal population. Governor Arthur Phillip said that about half of the Aboriginal people living around Sydney Cove died during the outbreak. Some later estimates have been higher,[86][87] though precise figures are hard to determine,[88] and Professors Carmody and Hunter argued in 2014 that the figure was more like 30%.[89] There is an ongoing debate, with links to the "history wars", concerning two main rival theories about how smallpox first entered the continent. (Another hypothesis suggested that the French brought it in 1788, but the timeline does not fit.) The central hypotheses of these theories suggest that smallpox was transmitted to Indigenous Australians by either:[86]

In 1914, Dr J. H. L. Cumpston, director of the Australian Quarantine Service tentatively put forward the hypothesis that smallpox arrived with British settlers.[90] Cumpston's theory was most forcefully reiterated by the economic historian Noel Butlin, in his book Our Original Aggression (1983).[91] Likewise David Day, in Claiming a Continent: A New History of Australia (2001), suggested that members of Sydney's garrison of Royal Marines may have attempted to use smallpox as a biological weapon in 1789.[92][full citation needed] However, in 2002, historian John Connor stated that Day's theory was "unsustainable".[93] That same year, theories that smallpox was introduced with settlers, deliberately or otherwise, were contested in a full-length book by historian Judy Campbell: Invisible Invaders: Smallpox and Other Diseases in Aboriginal Australia 1780–1880 (2002).[94] Campbell consulted, during the writing of her book, Frank Fenner, who had overseen the final stages of a successful campaign by the World Health Organization (WHO) to eradicate smallpox. Campbell argued that scientific evidence concerning the viability of variolous matter (used for inoculation) did not support the possibility of the disease being brought to Australia on the long voyage from Europe. Campbell also noted that there was no evidence of Aboriginal people ever having been exposed to the variolous matter, merely speculation that they may have been. Later authors, such as Christopher Warren,[95] and Craig Mear [96] continued to argue that smallpox emanated from the importation of variolous matter on the First Fleet. Warren (2007) suggested that Campbell had erred in assuming that high temperatures would have sterilised the British supply of smallpox.[95] H. A. Willis (2010), in a survey of the literature discussed above, endorsed Campbell's argument.[97] In response, Warren (2011) suggested that Willis had not taken into account research on how heat affects the smallpox virus, cited by the WHO.[98] Willis (2011) replied that his position was supported by a closer reading of Frank Fenner's report to the WHO (1988) and invited readers to consult that report online.[36][99]

The rival hypothesis, that the 1789 outbreak was introduced to Australia by visitors from Makassar, came to prominence in 2002, with Judy Campbell's book Invisible Invaders.[94] Campbell expanded upon the opinion of C. C. Macknight (1986), an authority on the interaction between indigenous Australians and Makassans.[100] Citing the scientific opinion of Fenner (who wrote the foreword to her book) and historical documents, Campbell argued that the 1789 outbreak was introduced to Australia by Makassans, from where it spread overland.[94] Nevertheless, Michael Bennett in a 2009 article in Bulletin of the History of Medicine, argued that imported "variolous matter" may have been the source of the 1789 epidemic in Australia.[101] In 2011, Macknight re-entered the debate, declaring: "The overwhelming probability must be that it [smallpox] was introduced, like the later epidemics, by [Makassan] trepangers on the north coast and spread across the continent to arrive in Sydney quite independently of the new settlement there".[102] Warren (2013) disputed this, on the grounds that: there was no suitable smallpox in Makassar before 1789; there were no trade routes suitable for transmission to Port Jackson; the theory of a Makassan source for smallpox in 1789 was contradicted by Aboriginal oral tradition; and, the earliest point at which there was evidence of smallpox entering Australia with Makassan visitors was around 1824.[103] Public health expert Mark Wenitong, a Kabi Kabi man, and John Maynard, Emeritus professor of Aboriginal History at the University of Newcastle agree that this is highly unlikely, with the added obstacle of very low population density between the north coast and Sydney Cove.[86]

A quite separate third theory, endorsed by the pathologist Dr G. E. Ford, with support from the academics Curson, Wright and Hunter, holds that the deadly disease was not smallpox but the far more infectious chickenpox, to which the Eora Aboriginal peoples had no resistance.[104] John Carmody, then at the University of Sydney School of Medical Sciences, suggested in 2010 that the epidemic was far more likely to have been chickenpox, as none of the European colonists were threatened by it, as he would have expected to happen.[105] However Wenitong and Maynard continue to believe that there is strong evidence that it was smallpox.[86]

Another major outbreak was observed in 1828–1830, near Bathurst, New South Wales.[90] A third epidemic occurred in the Northern Territory and northern Western Australia from the mid-1860s,[97] until at least 1870.

Polynesia

[edit]

Elsewhere in the Pacific, smallpox killed many indigenous Polynesians.[106] Nevertheless, Alfred Crosby, in his major work, Ecological Imperialism: The Biological Expansion of Europe, 900-1900 (1986) showed that in 1840 a ship with smallpox on it was successfully quarantined, preventing an epidemic amongst Māori of New Zealand. The only major outbreak in New Zealand was a 1913 epidemic, which affected Māori in northern New Zealand and nearly wiped out the Rapa Nui of Easter Island (Rapa Nui), was reported by Te Rangi Hiroa (Dr Peter Buck) to a medical congress in Melbourne in 1914.[107]

Micronesia

[edit]

The whaler ship Delta brought smallpox to the Micronesian island of Pohnpei on 28 February 1854. The Pohnpeians reacted by first feasting their offended spirits and then resorted hiding. The disease eventually wiped out more than half the island's population. The deaths of chiefs threw Pohnpeian society into disarray, and the people started blaming the God of the Christian missionaries. The Christian missionaries themselves saw the epidemic as God's punishment for the people and offered the natives inoculations, though often withheld such treatment from the priests. The epidemic abated in October 1854.[108]

Eradication

[edit]

Early in history, it was observed that those who had contracted smallpox once were never struck by the disease again. Thought to have been discovered by accident, it became known that those who contracted smallpox through a break in the skin in which smallpox matter was inserted received a less severe reaction than those who contracted it naturally. This realization led to the practice of purposely infecting people with matter from smallpox scabs in order to protect them later from a more severe reaction. This practice, known today as variolation, was first practiced in China in the 10th century.[36] Methods of carrying out the procedure varied depending upon location. Variolation was the sole method of protection against smallpox other than quarantine until Edward Jenner's discovery of the inoculating abilities of cowpox against the smallpox virus in 1796. Efforts to protect populations against smallpox by way of vaccination followed for centuries after Jenner's discovery. It is thought that smallpox has been eradicated since 1979.

Variolation

[edit]

The word variolation is synonymous with inoculation, insertion, en-grafting, or transplantation. The term is used to define insertion of smallpox matter, and distinguishes this procedure from vaccination, where cowpox matter was used to obtain a much milder reaction among patients.[36]

Asia

[edit]

The practice of variolation (also known as inoculation) first came out of East Asia.[109] First writings documenting variolation in China appear around 1500. Scabs from smallpox victims who had the disease in its mild form would be selected, and the powder was kept close to body temperature by means of keeping it close to the chest, killing the majority of the virus and resulting in a more mild case of smallpox.[110] Scabs were generally used when a month old, but could be used more quickly in hot weather (15–20 days), and slower in winter (50 days). The process was carried out by taking eight smallpox scabs and crushing them in a mortar with two grains of Uvularia grandiflora in a mortar.[36] The powder was administered nasally through a silver tube that was curved at its point, through the right nostril for boys and the left nostril for girls.[110] A week after the procedure, those variolated would start to produce symptoms of smallpox, and recovery was guaranteed. In India, where the European colonizers came across variolation in the 17th century, a large, sharp needle was dipped into the pus collected from mature smallpox sores. Several punctures with this needle were made either below the deltoid muscle or in the forehead, and then were covered with a paste made from boiled rice.[110] Variolation spread farther from India to other countries in south west Asia, and then to the Balkans.[36]

Mauritius

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In 1792 a slave-ship arrived on the French Indian Ocean island of Île de France (Mauritius) from South India, bringing with it smallpox. As the epidemic spread, a heated debate ensued over the practice of inoculation. The island was in the throes of revolutionary politics and the community of French colonists were acutely aware of their new rights as ‘citizens’. In the course of the smallpox epidemic, many of the political tensions the period came to focus on the question of inoculation, and were played out on the bodies of slaves. Whilst some citizens asserted their right, as property owners, to inoculate the slaves, others, equally vehemently, objected to the practice and asserted their right to protect their slaves from infection. Eighteenth-century colonial medicine was largely geared to keeping the bodies of slaves and workers productive and useful, but formal medicine never had a monopoly. Slaves on Île de France brought with them a rich array of medical beliefs and practices from Africa, India, and Madagascar. We have little direct historical evidence for these, but we do know that many slaves came from areas in which forms of smallpox inoculation were known and practiced.[111]

Europe

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Lady Mary Wortley Montagu with her son, Edward Wortley Montagu, who was variolated in March, 1718 by Dr. Charles Maitland[112]

In 1713, Lady Mary Wortley Montagu's brother died of smallpox; she too contracted the virus two years later at the age of twenty-six, leaving her badly scarred.[113] When her husband was made ambassador to Ottoman Empire, she accompanied him to Constantinople. It was here that Lady Mary first came upon variolation.[114] Two Greek women made it their business to engraft people with pox that left them un-scarred and unable to catch the pox again.[113] In a letter, she wrote that she intended to have her own son undergo the process and would try to bring variolation into fashion in England.[115] Her son underwent the procedure, which was performed by Charles Maitland,[113] and survived with no ill effects. When an epidemic broke out in London following her return, Lady Mary wanted to protect her daughter from the virus by having her variolated as well. Maitland performed the procedure, which was a success.[113] The story made it to the newspapers and was a topic for discussion in London salons.[114] Princess Caroline of Wales wanted her children variolated as well but first wanted more validation of the operation. She had both an orphanage and several convicts variolated before she was convinced.[109][113] When the operation, performed by the King's surgeon, Claudius Amyand, and overseen by Maitland,[110] was a success, variolation got the royal seal of approval and the practice became widespread.[116] When the practice of variolation set off local epidemics and caused death in two cases, public reaction was severe. Minister Edmund Massey, in 1772, called variolation dangerous and sinful, saying that people should handle the disease as the biblical figure Job did with his own tribulations, without interfering with God's test for mankind.[110][114] Lady Mary still worked at promoting variolation but its practice waned until 1743.[114]

Robert and Daniel Sutton further revived the practice of variolation in England by advertising their perfect variolation record, maintained by selecting patients who were healthy when variolated and were cared for during the procedure in the Sutton's own hygienic hospital.[114] Other changes that the Suttons made to carrying out the variolation process include reducing and later abolishing the preparatory period before variolation was carried out, making more shallow incisions to distribute the smallpox matter, using smallpox matter collected on the fourth day of the disease, where the pus taken was still clear, and recommending that those inoculated get fresh air during recovery.[110] The introduction of the shallower incision reduced both complications associated with the procedure and the severity of the reaction.[112] The prescription of fresh air caused controversy about Sutton's method and how effective it was in reality when those inoculated could walk about and spread the disease to those that had never before experienced smallpox.[110] It was the Suttons who introduced the idea of mass variolation of an area when an epidemic broke out as means of protection to the inhabitants in that location.[112]

News of variolation spread to the royal families of Europe. Several royal families had themselves variolated by English physicians claiming to be specialists. Recipients include the family of Louis XV following his own death of smallpox, and Catherine the Great, whose husband had been horribly disfigured by the disease.[114][116] Catherine the Great was variolated by Thomas Dimsdale, who followed Sutton's method of inoculation.[110] In France, the practice was sanctioned until an epidemic was traced back to an inoculation. After this instance, variolation was banned within city limits. These conditions caused physicians to move just outside the cities and continue to practice variolation in the suburbs.[114]

Edward Jenner
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Edward Jenner (1749–1823)

Edward Jenner was variolated in 1756 at age eight in an inoculation barn in Wotton-under-Edge, England. At this time, in preparation for variolation children were bled repeatedly and were given very little to eat and only given a diet drink made to sweeten their blood. This greatly weakened the children before the actual procedure was given.[110][117] Jenner's own inoculation was administered by a Mr. Holbrow, an apothecary. The procedure involved scratching the arm with the tip of a knife, placing several smallpox scabs in the cuts and then bandaging the arm. After receiving the procedure, the children stayed in the barn for several weeks to recover. First symptoms occurred after one week and usually cleared up three days later. On average, it took a month to fully recover from the encounter with smallpox combined with weakness from the preceding starvation.[114]

At the age of thirteen, Jenner was sent to study medicine in Chipping Sodbury with Daniel Ludlow, a surgeon and apothecary, from 1762 to 1770[113][117][118] who had a strong sense of cleanliness which Jenner learned from him. During his apprenticeship, Jenner heard that upon contracting cowpox, the recipient became immune to smallpox for the remainder of their life.[113][116] However, this theory was dismissed because of several cases proving that the opposite was true.[citation needed]

After learning all he could from Ludlow, Jenner apprenticed with John Hunter in London from 1770 to 1773.[113][118] Hunter was a correspondent of Ludlow's, and it is likely that Ludlow recommended Jenner to apprentice with Hunter. Hunter believed in deviating from the accepted treatment and trying new methods if the traditional methods failed. This was considered unconventional medicine at the time and had a pivotal role in Jenner's development as a scientist.[114][119]

After two years of apprenticeship, Jenner moved back to his hometown of Berkeley in Gloucestershire,[118] where he quickly gained the respect of both his patients and other medical professionals for his work as a physician.[114] It was during this time that Jenner revisited the connection between cowpox and smallpox.[113] He began investigating dairy farms in the Gloucestershire area looking for cowpox. This research was slow going as Jenner often had to wait months or even years before cases of cowpox would again return to the Gloucestershire area.[114] During his study, he found that cowpox was actually several diseases that were similar in nature but were distinguishable through slight differences, and that not all versions had the capacity to make one immune from smallpox upon contraction.[113]

Through his study, he incorrectly deduced that smallpox and cowpox were all the same disease, simply manifesting themselves differently in different animals, eventually setting back his research and making it difficult to publish his findings. Though Jenner had seen cases of people becoming immune to smallpox after having cowpox, too many exceptions of people still contracting smallpox after having had cowpox were arising. Jenner was missing crucial information which he later discovered in 1796.[114] Jenner hypothesized that in order to become immune to smallpox using cowpox, the matter from the cowpox pustules must be administered at maximum potency; else it was too weak to be effective in creating immunity to smallpox. He deduced that cowpox was most likely to transfer immunity from smallpox if administered at the eighth day of the disease.[113]

On May 14, 1796, he performed an experiment in which he took pus from a sore of a cowpox-infected milkmaid named Sarah Nelmes, and applied it to a few small scratches on the arm of an eight-year-old boy who had never before contracted either smallpox or cowpox, named James Phipps. Phipps recovered as expected.[113] Two months later, Jenner repeated the procedure using matter from smallpox, observing no effect. Phipps became the first person to become immune to smallpox without ever actually having the disease. He was variolated many more times over the course of his life to prove his immunity.[114]

Photograph by Allan Warner of two boys who came in contact with smallpox, 1901. The one on the right was vaccinated in infancy, the other was not vaccinated.

When the next cowpox epidemic broke out in 1798, Jenner conducted a series of inoculations using cowpox, all of them successful[113] including on his own son Robert.[114] Because his findings were revolutionary and lacked in evidence, the Royal Society (of which Jenner was a member) refused to publish his findings.[114] Jenner then rode to London and had his book An Inquiry into the Causes and Effects of the Variolæ Vaccinæ published by Sampson Low's firm[117] in June 1798.[120] The book was an instant bestseller among the elite in London salons, in the medical establishment and among the ladies and gentlemen of the enlightenment.[114]

Knowledge of the ability of cowpox to provide immunity from smallpox was present in England before Jenner's discovery. In 1774, a cattle dealer named Benjamin Jesty had successfully inoculated his wife and three sons using cowpox. This was before Jenner discovered the immunization capabilities of cowpox.[116] However, Jesty simply performed the procedure; he did not take the discovery any further by inoculating his family with smallpox matter to see if there would be a reaction or perform any other trials.[110] Jenner was the first to prove the effectiveness of vaccination with cowpox using scientific experimentation.[114]

United States

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Benjamin Franklin, who had lost his own son to smallpox in 1736, made the suggestion to create a pamphlet to distribute to families explaining how to inoculate their children themselves, so as to eliminate cost as the factor in the decision to choose to inoculate children. William Heberden, a friend of Franklin's and an English physician, followed through with Franklin's idea, printing 2000 pamphlets in 1759 which were distributed by Franklin in America.[110]

An American physician, John Kirkpatrick, upon his visit to London in 1743, told of an instance where variolation stopped an epidemic in Charleston, South Carolina, in 1738,[121] where 800 people were inoculated and only eight deaths occurred.[110] His account of the success of variolation in Charleston helped to play a role in the revival of its practice in London. Kirkpatrick also advocated inoculating patients with matter from the sores of another inoculated person, instead of using matter from the sore of a smallpox victim, a procedure that Maitland had been using since 1722.[112]

In 1832 President Andrew Jackson signed Congressional authorization and funding to set up a smallpox vaccination program for Indian tribes. The goal was to eliminate the deadly threat of smallpox to a population with little or no immunity, and at the same time exhibit the benefits of cooperation with the government.[122] In practice there were severe obstacles. The tribal medicine men launched a strong opposition, warning of white trickery and offering an alternative explanation and system of cure. They taught that the affliction could best be cured by a sweat bath followed by a rapid plunge into cold water.[123][124] Furthermore the vaccines often lost their potency when transported and stored over long distances with primitive storage facilities. It was too little and too late to avoid the great smallpox epidemic of 1837 to 1840 that swept across North America west of the Mississippi, all the way to Canada and Alaska. Deaths have been estimated in the range of 100,000 to 300,000, with entire tribes such as the Mandans wiped out.[125]

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Taterapox (which infects rodents) and camelpox are the closest relatives to smallpox, and share the same common ancestor with smallpox about 4,000 years ago. It is not clear exactly when during this period Variola first infected humans.[126] Cowpox, horsepox, and monkeypox are more distantly related. All of these viruses share a common ancestor about 10,000 years ago.[2] All of these viruses belong to the genus Orthopoxvirus.

See also

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References

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Sources

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  • Fenner, Frank; Henderson, Donald A.; Arita, Isao; Jezek, Zdenek; Ladnyi, Ivan Danilovich; Organization, World Health (1988). Smallpox and its eradication. World Health Organization. hdl:10665/39485. ISBN 978-92-4-156110-5. online
  • Hopkins, Donald R. The Greatest Killer: Smallpox in History (U of Chicago Press, 2002)
  • Patterson, Kristine B. and Thomas Runge, "Smallpox and the native American." American journal of the medical sciences 323.4 (2002): 216–22. online

Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia
from Grokipedia
The history of smallpox chronicles the trajectory of a highly contagious, often lethal disease caused by the variola virus, which afflicted human populations for at least three millennia, inflicting recurrent epidemics with case-fatality rates up to 30% for the predominant variola major strain, until its global eradication was certified in 1980. Evidence of the disease dates to , where pockmarked rashes on mummies, including that of Ramses V around 1157 BCE, provide the earliest physical indications of variola infection. Over centuries, smallpox spread via trade routes, warfare, and migration, devastating societies worldwide; it contributed to the collapse of empires, such as in the following European contact, and is estimated to have caused 300–500 million deaths in the alone. Pre-modern control relied on variolation, an empirical technique originating in Asia and Africa—possibly as early as the 10th century in China or India—entailing controlled exposure to smallpox scabs or pustules to provoke mild illness and subsequent immunity, though with risks of full disease transmission and a 1–2% mortality rate. This practice reached Europe in the early 18th century via figures like Lady Mary Wortley Montagu, who observed it in the Ottoman Empire and advocated its use in Britain. A pivotal advancement occurred in 1796 when English physician Edward Jenner demonstrated that inoculation with cowpox—a related, milder poxvirus—conferred immunity to smallpox, founding modern vaccinology and enabling safer, scalable prophylaxis. The 20th century saw intensified global efforts, culminating in the World Health Organization's 1967 campaign employing ring vaccination and surveillance to interrupt transmission chains, eliminating the disease from and the by the early 1950s and by 1975. The last known natural case occurred in in 1977, after which laboratory containment and verification confirmed no ongoing circulation, leading to the 1980 declaration of eradication—the first and only for any human infectious disease—averting an estimated annual toll of millions in prior endemic eras.

Origins and Early Evidence

Archaeological, Genetic, and Textual Records

The earliest physical evidence of derives from pockmark lesions visible on the mummified skin of Pharaoh Ramses V, who died around 1157 BCE during Egypt's 20th Dynasty. These raised, pustular scars on the pharaoh's face and body, preserved through mummification, align with the characteristic rash of variola major, though direct viral confirmation via remains absent due to degradation. No earlier mummified cases show such definitive lesions, underscoring Ramses V as the oldest archaeological indicator of the disease's presence in human remains. Genetic analyses of variola virus (VARV) genomes from historical samples reveal its divergence from camelpox virus (CMLV), its closest relative, approximately 6,000 years ago, around 4000 BCE, during the era of intensified animal and in the and . This timeline correlates with increased human-animal contact facilitating zoonotic jumps, though VARV's strict human host adaptation suggests subsequent specialization. Broader phylogenies posit an ancestral split from African rodent-associated poxviruses (e.g., taterapox) between 16,000 and 68,000 years ago, but the emergence of epidemic-capable strains postdates this, aligning with early agrarian societies rather than hunter-gatherers. Recent sequencing of 17th-century and Viking-era VARV strains confirms lineage extinctions and a around 1,700 years ago, indicating dynamic evolution but ancient roots. Ancient textual records provide symptomatic descriptions interpretable as smallpox. In India, the Sushruta Samhita, compiled circa 600 BCE, details "masurika" as eruptions resembling lentil seeds (masura) on the skin, accompanied by fever and pustules, marking one of the earliest written accounts of a pox-like illness in Ayurvedic literature. Scholars correlate these features—eruptive boils, contagion, and lethality—with variola, though the texts blend it with other dermatoses like measles. Chinese annals from the Han Dynasty (circa 200 BCE) reference "tianhua" (heavenly flowers), evoking rash-like blossoms with prodromal fever and pustulation, though systematic disease recognition solidified later in the Jin Dynasty (circa 300 CE) via Ge Hong's Elbow Reserve, which explicitly notes epidemic spread and scarring. These accounts, while not naming variola explicitly, prefigure clinical hallmarks confirmed by modern epidemiology, distinguishing smallpox from contemporaneous afflictions like varicella.

Initial Emergence and Zoonotic Hypotheses

The (VARV), causative agent of , is hypothesized to have originated via a zoonotic spillover from reservoirs, with phylogenetic analyses indicating divergence from African poxviruses such as taterapox virus (TATV), isolated from the pouched Gerbilliscus kempi. This jump likely occurred amid increased human- interactions facilitated by early agricultural settlements and animal during the period (approximately 10,000–6,000 years ago), which promoted denser populations and grain storage attracting . estimates place the separation of VARV from TATV and related poxvirus (CMLV) lineages at around 3,400–7,700 years ago, aligning with urbanization in the and rather than the initial farming onset. Empirical phylogenetic trees, constructed from full-genome sequences of orthopoxviruses, position VARV within a distinct from broader virus (CPXV) strains, refuting notions of direct derivation from cattle poxviruses despite superficial morphological similarities in lesions. Instead, VARV's closest relatives are TATV and CMLV, both associated with or arid-adapted mammalian hosts, supporting a event followed by rapid loss that conferred strict tropism—evident in deletions of host-range like ANK and KILR, enabling efficient person-to-person transmission without animal intermediaries. This post-spillover specialization, estimated to have stabilized VARV as an obligate by 3,000–4,000 years ago, correlates with the virus's genomic stability and lack of enzootic reservoirs today. No definitive archaeological or genetic evidence confirms VARV circulation prior to the or early ; claims of pre-Neolithic infections, inferred from ambiguous paleopathological lesions in ancient remains, lack viral DNA confirmation and are contradicted by molecular showing after 4,000 years ago. Earlier estimates suggesting divergence 16,000–68,000 years ago, based on uncalibrated clocks tied to sparse historical records, have been revised downward with refined substitution rates (approximately 2 × 10⁻⁶ per site per year in conserved regions), emphasizing causal factors like over deep-time transmission, which would require unsustainable thresholds in sparse groups. Thus, VARV's sustained pathogenicity arose from ecological pressures of , not primordial zoonoses.

Pre-Modern Epidemics and Spread

Eurasian and African Outbreaks

The , originating around 165 CE among Roman troops returning from campaigns in the East, exhibited symptoms described by the physician —including fever, headache, and a vesicular rash progressing from face to extremities—that align with variola major , though definitive pathogen identification awaits recovery. The outbreak ravaged the over 15 years, claiming an estimated 5 to 10 million lives—roughly 10% of the population of 50 to 60 million—exacerbated by dense concentrations, urban trade hubs like and , and disrupted supply lines that hindered containment. mobility along frontier roads and sea routes from facilitated rapid dissemination, with recurrence in affected regions underscoring the virus's capacity for sustained transmission in Eurasian networks. In the early medieval period, smallpox persisted endemically across , with textual records from the providing the earliest systematic differentiation of the disease. The Persian physician Rhazes (Muhammad ibn Zakariya al-Razi), writing circa 910 CE in his treatise A Treatise on the Small-Pox and , cataloged clinical features such as pustular eruptions sparing the face initially, high fever, and , distinguishing variola from based on distribution and patient age patterns. These accounts reflect outbreaks fueled by caravan trade across the , urban growth in and , and pilgrimage circuits like the , which aggregated susceptible populations and amplified person-to-person spread. Mortality in unvaccinated groups averaged 20 to 30% for variola major strains, with survivors frequently exhibiting permanent facial scarring from coalesced pustules, as noted in Byzantine and Arabic chronicles. Medieval Eurasian epidemics intensified through warfare and commerce, as Mongol invasions from the 13th century onward propelled the virus westward via steppe nomad armies and disrupted quarantines. In , outbreaks recurred amid feudal conflicts and Crusader movements, with records from 11th- to 14th-century and documenting child-heavy mortality waves that depleted rural labor amid trade fairs and sieges. Transmission vectors included overland routes from and Mediterranean ports, where shipboard crowding mirrored later patterns, sustaining cycles in populations lacking . In , smallpox circulated endemically in sub-Saharan regions by the first millennium CE, with intensified outbreaks from the linked to and the Atlantic slave trade's coastal entrepôts. European slavers inadvertently amplified dissemination by funneling infected individuals from West African interiors—where the virus thrived in kinship villages and markets—to ports like and , fostering recurrent epidemics with case-fatality rates of 20 to 30% among non-immune groups. Survivor pock-marking served as epidemiological markers in oral histories, while caravan and warrior migrations southward into the extended the disease's footprint pre-colonially.

Introduction to the Americas and Demographic Impacts

Smallpox reached the Americas in 1519, carried by members of Hernán Cortés's expedition from the Caribbean, where the disease had already spread from earlier Spanish contacts. The ensuing epidemic ravaged the Aztec Empire, peaking in 1520 and killing an estimated 25% of Tenochtitlán's population, including Emperor Cuitláhuac on December 4, 1520, which critically weakened Aztec leadership and military cohesion during the Spanish siege. This disruption facilitated Cortés's conquest by 1521, as the disease's rapid spread through immunologically naive populations—lacking prior exposure or genetic adaptations—impaired societal functions like governance and warfare, independent of direct Spanish military superiority. In virgin soil epidemics, smallpox mortality among indigenous groups often exceeded 90%, as evidenced by missionary accounts from the onward and archaeological findings of mass graves with pockmarked skeletal remains confirming variola virus infection.34481-5/pdf) Such rates stemmed from causal factors including absent , limited genetic resistance alleles prevalent in populations, and minimal pre-existing medical knowledge for containment, leading to near-total community annihilation in isolated groups. Overall demographic collapse in the post-1492, with indigenous populations declining by up to 95% within centuries, was predominantly driven by introduced pathogens like , outpacing other factors such as warfare or enslavement in scale and speed. Subsequent outbreaks amplified these impacts, such as the 1616–1619 epidemic along the New England coast, which reduced some Native populations by 75–90% through unchecked transmission via trade and exploration contacts. In the Southeast, recurrent waves from 1698 to 1715, including smallpox intertwined with the Yamasee War, decimated tribes like the Catawba and Yamasee, eroding resistance to colonial encroachment. Farther north, the 1775–1782 epidemic, spreading via military campaigns around Quebec, inflicted at least 50% mortality on affected indigenous bands, further tilting demographic balances toward European settlers by disrupting alliances and subsistence economies. These patterns underscore smallpox's role in enabling sustained European dominance through repeated, asymmetric demographic shocks rather than singular events.

Pacific Islands and Australia

Smallpox reached following the arrival of the in January 1788, with the initial outbreak manifesting in April 1789 among Aboriginal communities near . The disease spread rapidly through immunologically naive populations, causing high mortality; estimates indicate it killed approximately half of the affected Aboriginal groups in the region, leaving bodies unburied along shorelines and exacerbating early colonial tensions. Historical accounts attribute the introduction to inadvertent transmission from infected Europeans or contaminated goods, though debates persist among scholars regarding whether the illness was definitively , citing limited spread to colonists and atypical symptom reports that suggest possible alternative viral agents with similar lethality in isolated groups. In the Pacific Islands, smallpox arrived primarily in the 19th century via European whalers, traders, and ships from endemic regions like and , exploiting the isolation of Polynesian and Micronesian societies. The 1853-1854 epidemic in , imported to from , infected over 10,000 individuals and killed 5,000 to 7,000 in a population of roughly 70,000, representing a case-fatality rate exceeding 50% among the unexposed. measures, including ship inspections and isolation camps, proved inadequate against rapid community transmission, as the virus evaded early detection and spread inland despite prohibitions on inter-island travel. Comparable devastation struck other islands, such as the in 1854, where claimed about 2,000 lives out of a population of 5,000, and in 1856, with roughly one-third of inhabitants perishing. These outbreaks, often seeded by transient vessels bypassing rudimentary port controls, triggered profound cultural disruptions: surviving populations faced labor shortages, breakdown of kinship networks, and accelerated European influence amid demographic collapse, with mortality rates in naive groups reaching 30-50% due to variola major's . In , lingered on remote frontiers into the early 20th century, with isolated cases among Aboriginal communities reported as late as , underscoring persistent vulnerabilities despite campaigns targeting settler populations.

Traditional Inoculation Practices

Variolation Techniques and Regional Variations

Variolation involved the deliberate introduction of smallpox virus material into healthy individuals to provoke a milder infection and subsequent immunity, with practices emerging independently in multiple regions. In China, the method dates to at least the 11th century, though the earliest documented description appears in a 1549 text; practitioners dried smallpox scabs and insufflated the powder into the nostrils of recipients, aiming to induce a controlled case of the disease. Similar nasal insufflation techniques were employed in India around the same period, potentially involving cutaneous scarification as a variant, reflecting early empirical observations that prior mild exposure reduced severe outcomes. In Africa, including Sudanese regions, variolation by the 17th century utilized comparable nasal methods, where dried scabs or pustule fluid were blown into the nose, often by specialized inoculators who selected material from mild cases to minimize risks. Ottoman practitioners in the 17th century refined skin-based techniques, scratching superficial incisions on the arm and inserting pus or scab matter from recovered patients, a method observed and documented by European travelers. Lady Mary Wortley Montagu, during her residence in Constantinople from 1716 to 1718, witnessed these "ingrafting" procedures performed on children and reported their routine use among the populace, noting the selection of low-virulence strains to achieve milder symptoms. Across regions, reduced mortality to approximately 1-2%—compared to 20-30% in natural infections—by transmitting live variola virus in diluted form, which empirical records from inoculated communities substantiated through lower fatalities. However, the technique's limitations included variable due to inconsistent viral dosing and host factors, with inoculated individuals remaining contagious and occasionally sparking outbreaks, as the induced mirrored natural smallpox in transmissibility albeit with attenuated severity. Regional adaptations, such as Ottoman emphasis on arm abrasions over nasal routes, likely arose from local healer traditions prioritizing accessibility and observed success rates, though all methods carried inherent risks of full-blown in 1-2% of cases.

Efficacy, Risks, and Limitations

Variolation typically induced a milder form of , with case-fatality rates of 1-2% compared to 20-30% for natural infection, thereby reducing individual mortality risk by a factor of 10 to 30 while conferring lasting immunity upon recovery. This stemmed from controlled exposure to live , often via nasal of scabs or intradermal insertion of pustular material, which replicated the disease process at lower viral loads in many cases. Historical records from regions like and the documented survival rates supporting this attenuation, though outcomes varied by viral strain and host factors. Despite individual protection, failed to interrupt transmission chains, as inoculated individuals shed infectious for days to weeks, exposing unprotected contacts to unmodified, potentially lethal strains. This contagiousness precipitated secondary outbreaks among household members and communities unless entire groups were simultaneously isolated and treated, undermining herd-level control and perpetuating epidemics even in -practicing societies. from 18th-century showed recurrent waves of despite increasing variolation adoption, as the method amplified localized spread without eradicating reservoirs. Risks extended beyond direct mortality, including unintended full-blown if virulent material was used and secondary bacterial super from open pustules, which exacerbated scarring, , or death in vulnerable patients. Sourcing variolar matter from survivors posed hazards, as mild-case scabs could harbor unpredictable , occasionally yielding fatality rates exceeding 5% in uncontrolled applications. Ethical concerns over deliberate fueled rejection, exemplified by the 1721 epidemic where physician Zabdiel Boylston's of 248 persons yielded only 6 deaths (2.4% rate), yet sparked public and clerical backlash for risking community transmission and defying providence, with opponents citing imported cases traceable to variolated ships. Limitations in further constrained variolation's role in mass eradication, requiring of recipients to avert iatrogenic epidemics and dependence on scarce, low-virulence sources that were logistically challenging to procure at population scale. Without standardized , the practice remained artisanal and regionally variable, incapable of achieving the uniform immunity needed for sustained outbreak suppression, as evidenced by persistent 18th-century epidemics in inoculated populations.

Invention and Evolution of Vaccination

Edward Jenner's Cowpox Method

In May 1796, English physician Edward Jenner conducted the first documented experiments with cowpox material in Berkeley, Gloucestershire, building on longstanding rural observations that milkmaids exposed to cowpox rarely contracted smallpox. On May 14, he inoculated the arm of eight-year-old James Phipps with pus extracted from cowpox lesions on the hand of Sarah Nelmes, a local milkmaid infected from her cow, Blossom. Phipps developed a mild local pustule and brief fever but recovered quickly, demonstrating the attenuated nature of cowpox compared to variola virus. Jenner then tested for cross-protection by variolating Phipps—exposing him to live smallpox material—on July 1, 1796; no smallpox symptoms appeared, confirming immunity. He repeated the process multiple times on Phipps with consistent results, and extended arm-to-arm human passages of cowpox material to additional subjects, including adults and infants, vaccinating at least 23 individuals by 1798. These trials showed uniform mild reactions—typically vesicular lesions without systemic illness—and complete resistance to subsequent smallpox challenge via variolation, contrasting sharply with variolation's 1-2% mortality rate and risk of spreading unmodified virus. In his 1798 publication, An Inquiry into the Causes and Effects of the Variolae Vaccinae, Jenner detailed these empirical findings, distinguishing "true" —derived from equine origins and transmissible to cows—as the effective agent, while dismissing spurious variants lacking protective . The virus, later termed (from Latin vacca, cow), proved non-pathogenic in healthy humans beyond localized , inducing cross-protective antibodies against variola without the disease's lethality. Early data from Jenner's controlled challenges indicated 100% protection in vaccinated subjects, establishing as a safer alternative to through direct causal demonstration rather than anecdotal reliance.

Global Dissemination and Refinements

Following Jenner's 1796 demonstration, smallpox disseminated across and through state-sponsored programs and private initiatives, with of reduced mortality prompting mandates. In Britain, the Vaccination Act of 1853 rendered infant compulsory, correlating with a sharp decline in annual smallpox deaths from over 2,000 in the to fewer than 200 by the 1890s. In the United States, enacted the first mandate in 1809, followed by school-entry requirements in multiple states by the mid-19th century, which curbed urban epidemics and eliminated routine endemic transmission by the early . These policies, enforced amid logistical challenges like inconsistent potency and public resistance, achieved coverage rates exceeding 80% in compliant populations, averting tens of thousands of deaths annually in vaccinated regions. To mitigate risks from human arm-to-arm passage, such as bacterial or viral mutation, vaccine production standardized on calf lymph by the late , yielding purer, scalable supplies. British authorities adopted this method around 1898, with continental following suit earlier; Italy's Luigi Sacco and others refined calf inoculation techniques from the 1800s, enabling glycerin-preserved lymph for transport. This shift reduced adventitious infections and supported export to colonies, though early 20th-century global coverage lagged, remaining under 50% in many developing areas by 1950 due to refrigeration needs and uneven , permitting persistence in tropics. Technological refinements addressed stability hurdles, culminating in freeze-dried vaccines developed in the and scaled in the , which withstood heat without potency loss—critical for equatorial deployment. Soviet production supplied millions of doses, stabilizing supply chains and enabling campaigns that boosted coverage to approximately 20% globally by , averting an estimated 100-200 million deaths over the century relative to unvaccinated baselines. Vaccine complications, including vaccinia necrosum and post-vaccinial , occurred at rates of 1-3 deaths per million primary vaccinations, primarily in infants or immunocompromised individuals, prompting screening protocols but not undermining net benefits given variola major's 30% case-fatality rate. Data from U.S. (e.g., 68 complications-linked deaths from 1959-1966 amid millions vaccinated) informed risk-stratified policies, such as pausing routine use in low-risk areas while prioritizing high-incidence zones, reflecting causal trade-offs where vaccination's effects exceeded rare adverse events. Logistical refinements, including jet injectors for mass administration, further scaled delivery despite anti-mandate movements, as evidenced by Britain's 1853-1907 enforcement yielding 90% compliance in urban centers.

20th-Century Eradication Efforts

Pre-WHO Initiatives and Setbacks

In the early 1900s, routine vaccination programs in the United States and United Kingdom markedly reduced smallpox incidence through compulsory measures and public health enforcement. In the US, peak outbreaks occurred around 1901–1902, but sustained vaccination efforts thereafter confined cases to sporadic importations, with only isolated incidents reported by the 1930s. Similarly, in the UK, mass vaccination campaigns limited the disease to occasional events following external introductions. These successes demonstrated vaccination's efficacy in high-compliance settings but highlighted dependence on near-universal coverage, which proved elusive elsewhere. The League of Nations Health Organization in the explored coordinated international action, including a Japanese delegate's proposal at an International Sanitary Conference to designate an international scourge warranting global eradication efforts. However, these initiatives faltered due to inadequate funding, divergent national priorities, and logistical barriers, preventing scalable implementation. In Asia, hybrid approaches blending traditional with Jennerian offered partial suppression but were constrained by variolation's inherent risks, such as inducing unintended outbreaks, and inconsistent adoption, yielding only localized control rather than elimination. Resurgences marked the 1930s–1950s amid incomplete coverage and disruptions like World War II, with India experiencing endemic persistence and outbreaks affecting millions annually due to dense populations and vaccination shortfalls. The Soviet Union, by contrast, achieved domestic eradication through intensive, state-mandated vaccination drives by the mid-century, supplying freeze-dried vaccine that aided regional efforts. Colonial Africa saw persistent gaps, as European administrations prioritized urban or coastal areas, leaving rural regions vulnerable to recurrent epidemics owing to supply chain issues, resistance, and uneven enforcement from the 1920s onward. These disparities underscored empirical limits: vaccination curbed transmission where applied rigorously but failed globally without comprehensive, sustained logistics.

WHO Intensive Campaign Strategies

In 1967, the initiated the Intensified Eradication Programme amid an estimated 10 to 15 million annual cases globally, marking a pivot from prior mass efforts that had proven insufficient in densely populated endemic areas. The core tactic emphasized surveillance-containment, particularly ring , which focused on immunizing direct contacts of confirmed cases and their immediate networks to encircle and extinguish outbreaks, thereby conserving limited supplies compared to indiscriminate mass campaigns. This approach leveraged that vaccinating 80% of contacts could reliably interrupt transmission chains, prioritizing verifiable containment over broader immunization. Regional strategies diverged based on and infrastructure: in West and Central Africa, where endemic transmission persisted in rural pockets, initial mass drives supplemented to build baseline immunity, while in —particularly —and parts of , ring dominated for sporadic or imported cases, enabling rapid response without exhaustive coverage. These tactics collectively targeted thresholds of approximately 80% in high-risk zones, with containment proving causally effective in reducing case incidence through localized barriers to spread. Logistical innovations underpinned execution, including the widespread adoption of jet injectors in 1967 for efficient intradermal delivery without needles, which minimized vaccine waste by a factor of 2 to 3 and accelerated mass efforts where needed. The programme coordinated production of freeze-dried , with donations—especially from the —scaling to tens of millions of doses annually by the late , supported by quality testing to ensure potency. Vaccination teams incorporated gender-balanced compositions to foster trust and compliance in conservative communities, enhancing uptake amid cultural resistances.

Surveillance, Containment, and Final Cases

In the closing stages of the eradication campaign, relied on exhaustive case logging, verification of rash illnesses, and rapid to trace and interrupt transmission chains, with empirical records demonstrating the progressive contraction of outbreaks to isolated incidents. To counter underreporting driven by fear or stigma, programs introduced financial incentives, such as cash bounties awarded to individuals or villages for promptly notifying the first detected case in an area, which empirically increased reporting rates and accelerated detection in remote or resistant communities. In India, particularly during the 1975 Bihar operations, diagnostic challenges arose from frequent misidentification of severe chickenpox cases as smallpox, delaying containment as teams investigated false positives amid high chickenpox prevalence, thereby extending the timeline for declaring regions clear. The last verified indigenous case in India occurred on May 17, 1975, involving a child in Bihar's Katihar district, after which intensified searches yielded no further natural transmissions in the country. Shifting to Africa, the focus revealed the global finale: on October 26, 1977, , a 23-year-old hospital cook in , , developed variola minor symptoms after contact with an infected traveler, marking the last documented natural human case; he recovered following isolation and supportive care, with and vaccination rings confirming no secondary spread. Extensive post-event across remaining at-risk zones, including market inspections and household enumerations, logged zero additional natural cases, providing causal evidence of transmission cessation by late 1977. A subsequent laboratory incident in 1978 at the , UK, exposed medical photographer Janet Parker to aerosolized virus escaping via defective ventilation from a research lab below her workspace, resulting in her and death on September 11, 1978; immediate , of over 300 contacts, and facility contained the breach without further cases, though it highlighted lapses in handling residual virus stocks. This event, unrelated to natural circulation, reinforced the absence of endemic chains, as global logs post-1977 evidenced no sustained or wild-type transmissions.

Certification of Eradication and Lessons Learned

The Global Commission for the Certification of Smallpox Eradication, comprising independent scientists from 19 nations, convened in December 1979 to evaluate epidemiological data, reports, and search efforts worldwide, ultimately certifying that transmission had ceased globally with no evidence of human or animal reservoirs. This assessment relied on the absence of reported cases since October 1977, coupled with intensive verification activities including active searches in high-risk areas, analyses of suspected samples, and confirmation that variola virus did not persist subclinically or in nonhuman hosts, as subclinical infections were rare and non-transmissible without rash. The commission deemed the likelihood of undetected reservoirs negligible, based on the virus's strict human host specificity and the robustness of post-1977 systems capable of detecting even isolated chains of transmission. In May 1980, the 33rd formally accepted the commission's findings and declared smallpox eradicated, marking the first and only human infectious disease to achieve this status through deliberate intervention. This process underscored the causal efficacy of sustained, data-driven verification over mere absence of cases, with international commissions conducting country-specific audits from onward to rule out hidden foci in regions like and . Ongoing post- measures, including global reporting networks and reserve diagnostics, have maintained vigilance without detecting resurgence, empirically confirming the eradication's durability despite potential underreporting risks in less-monitored areas. Key lessons from the effort highlight the superiority of targeted ring vaccination—focusing resources on contacts of cases—over universal mass campaigns, which proved logistically inefficient and costly in the early phases; this adaptive strategy, implemented from , accelerated containment while minimizing vaccine doses needed. Political will from major donors, including substantial funding and technical support from the and totaling around $300 million, enabled cross-ideological cooperation essential for accessing endemic zones; the program has yielded economic benefits recovered over 130 times in global economies since 1980 through avoided vaccination, treatment, and mortality costs, with the United States recouping its contribution every 26 days via similar savings. Critiques have noted overreliance on voluntary compliance and in politically unstable or remote endemic regions, where cultural resistance or weak infrastructure could have concealed reservoirs, yet four decades of absence post-1980, verified through independent audits, refute resurgence claims and affirm the causal chain from intensified to global elimination. These insights emphasize empirical adaptability and international resolve as replicable for other pathogens lacking reservoirs, though success hinged on smallpox's unique visibility and vaccine stability.

Post-Eradication Developments

Retention and Destruction Debates for Virus Stocks

Following the World Health Organization's (WHO) certification of smallpox eradication in 1980, discussions emerged on the fate of remaining variola virus stocks, with initial plans in the 1980s targeting destruction in the early . In 1990, a WHO scientific advisory committee recommended destruction of all known stocks by December 31, 1993, to eliminate any risk of accidental release or misuse, but this deadline was postponed amid concerns over incomplete research on countermeasures. Further delays occurred, including a 1999 WHO resolution deferring destruction pending additional studies on antiviral agents and improvements. By the mid-, global stocks were consolidated into two secure repositories: the Centers for Disease Control and Prevention (CDC) in , , and the State Research Center of Virology and Biotechnology (Vector) in Koltsovo, , following Russia's 1994 transfer of its holdings to Vector. Proponents of retention argue that maintained stocks enable essential research for developing and refining therapeutics against potential reemergence, emphasizing empirical benefits over speculative risks. For instance, the antiviral (TPOXX), approved by the U.S. in 2018 under the animal rule for treatment, relied on variola virus studies to validate efficacy against , including and animal models informed by stock-derived data. Such research has advanced understanding of and host responses, supporting broader countermeasures without documented containment failures since eradication. Destruction advocates, including some WHO members, highlight theoretical accident risks in high-containment labs, yet shows zero laboratory-originated outbreaks post-1980, contrasting with pre-eradication incidents like the 1978 Birmingham escape. A 2014 discovery of six vials labeled "variola" in an FDA storage room at the National Institutes of Health campus underscored biosafety vulnerabilities but reinforced containment efficacy, as no breaches occurred and the vials—dating to around 1954—yielded viable virus upon testing without external exposure. This event prompted enhanced U.S. inventory protocols but did not precipitate outbreaks, empirically validating layered safeguards like BSL-4 protocols at authorized sites. Overall, retention's utility in fostering verifiable advancements, such as tecovirimat's mechanism targeting viral envelope formation, outweighs unsubstantiated destruction imperatives given the absence of post-eradication incidents.

Bioweapons Research and Historical Programs

During , Imperial Japan's , directed by Shiro Ishii, conducted extensive biological warfare research and field trials in occupied , including experiments with smallpox virus on human subjects to assess weaponization potential. These efforts aimed to develop deployable agents but were hampered by logistical challenges and the pathogen's sensitivity to environmental factors, with no evidence of large-scale operational use of smallpox specifically. The established a biological weapons program in 1943 under President , with research at Camp Detrick (later ) evaluating for aerosol dissemination among other agents from the onward. By 1969, following assessments of limited strategic value due to vaccine availability and eradication progress, President ordered the termination of offensive biological weapons development, destroying stockpiles while retaining capabilities for defensive research. The Soviet Union's program, expanded in the 1970s, achieved industrial-scale production of weaponized variants, reportedly yielding hundreds of tons of agent by the 1980s for missile and delivery. A 1971 field test on near Aralsk, , resulted in an accidental release, infecting at least 10 individuals (including laboratory personnel) and causing 3 deaths before containment through and . The program persisted covertly into the 1990s despite the 1972 , with defectors like confirming efforts to enhance and vaccine resistance. Smallpox's viability as a bioweapon faced inherent limitations, including instability—analogous to virus, with half-lives of approximately 6 hours at low humidity and 22°C but rapid decay in sunlight or higher moisture—restricting effective dissemination range and duration. Widespread infrastructure further undermined offensive utility, as immune populations reduced transmission chains, though unvaccinated targets remained vulnerable. No declassified evidence confirms verified offensive smallpox programs post-1980, with post-Cold War efforts shifting to defensive countermeasures.

Contemporary Risks, Preparedness, and Synthetic Biology Concerns

In 2017, researchers at the University of Alberta synthesized infectious horsepox virus, an orthopoxvirus relative of variola, using chemically synthesized DNA fragments totaling over 200 kilobases ordered commercially for approximately $100,000, assembled via recombination in yeast and propagated in cells, demonstrating the technical feasibility of reconstructing large DNA viruses from sequence data. The complete variola major genome, approximately 186 kilobases, has been publicly available since the 1990s through databases like NCBI, enabling in silico design but not simplifying physical reconstruction, which requires precise fragment assembly, error-free synthesis of repetitive sequences, and specialized biosafety level 4 facilities for propagation due to the virus's stability and infectivity. While tools like CRISPR-Cas9 facilitate targeted edits and could theoretically aid variola genome manipulation, empirical barriers remain substantial: off-target mutations, inefficient large-scale assembly (beyond 100 kb), and the need for orthopox-specific host factors limit non-state actors' capabilities, with no verified instances of synthetic variola production despite advances in DNA printing. Preparedness efforts center on vaccine stockpiles and response modeling, with the maintaining over 100 million doses of ACAM2000, a replication-competent vaccinia-based , in the , supplemented by non-replicating alternatives like JYNNEOS for broader deployment. Antivirals such as provide additional layers, targeting proteins. CDC and WHO-led tabletop exercises, building on frameworks like ring , indicate that early detection via enhanced surveillance could contain outbreaks through and post-exposure , with models projecting R0 values of 3-6 reducible below 1 via 80-90% coverage in affected rings, though logistical challenges in mass campaigns persist absent routine immunity. The 2022-2024 (clade IIb) outbreak, exceeding 100,000 confirmed cases globally and underscoring zoonotic potential, has reinforced surveillance needs, as prior confers partial cross-protection (e.g., 70-85% efficacy against ), but populations born post-1980 exhibit waning , elevating vulnerability to related poxviruses. No empirical evidence indicates active smallpox biothreats or clandestine variola programs in the , with WHO investigations dismissing rumors as misidentified orthopox cases; however, rogue actors with access to could pose asymmetric risks, mitigated by international genome screening protocols and dual-use oversight, though causal realism demands prioritizing verifiable containment over speculative panic.

Distinctions from Other Orthopoxviruses

Variola virus exhibits greater lethality than most other orthopoxviruses, with Variola major causing a case fatality rate (CFR) of approximately 30% in unvaccinated individuals, ranging from 15-50% depending on strain and population immunity, while Variola minor has a CFR of less than 1%. In comparison, cowpox virus typically produces mild, self-limiting infections in humans with rare fatalities, often localized to skin lesions without systemic spread. Monkeypox virus CFR varies from 1-10% historically, with lower rates in recent outbreaks, and features less severe respiratory involvement and more prominent lymphadenopathy absent in smallpox. A key epidemiological distinction is variola's exclusive adaptation to humans, lacking any known animal or environmental reservoir, which contrasts with zoonotic orthopoxviruses like cowpox and monkeypox that maintain reservoirs in rodents and other mammals, enabling sporadic spillovers. This human specificity facilitated targeted interruption of transmission chains, unlike the persistent zoonotic cycles in relatives. Transmission of variola relies solely on sustained human-to-human contact via respiratory droplets or fomites, without intermediate animal hosts, differing from the variable routes in cowpox (direct contact with infected animals) and monkeypox (rodent-to-human jumps). Genomically, variola possesses the smallest genome among orthopoxviruses at approximately 186 kbp, with deletions and mutations in genes related to host range and immune evasion that enhance human tropism and virulence, such as reduced functional orthologs for animal adaptation seen in broader-host viruses like cowpox. These features, including specific nucleotide substitutions in immunomodulatory genes, underpin differential diagnoses via PCR assays targeting variola-unique sequences, distinguishing it from morphologically similar orthopoxviruses. Historically, such distinctions were critical during differential diagnoses; for instance, early human monkeypox cases in the Democratic Republic of Congo in 1970, emerging amid smallpox surveillance, were initially probed as potential variola recrudescence but confirmed via virus isolation and serology as a separate entity due to milder progression and zoonotic links. This misattribution risk underscores variola's uniform pustular rash without prodromal nodes, aiding retrospective clarifications of pre-eradication outbreaks.

Evolutionary Relationships and Cross-Immunity Insights

The variola virus (VARV), causative agent of smallpox, belongs to the genus Orthopoxvirus within the family Poxviridae, sharing a common ancestor with other orthopoxviruses such as vaccinia virus (VACV), cowpox virus (CPXV), and monkeypox virus (MPXV). Phylogenetic analyses of VARV genomes reveal two primary clades: P1, encompassing variola major strains associated with higher mortality, and P2, comprising variola minor (alastrim) strains with lower virulence, diverging approximately 300–400 years ago based on molecular clock estimates from whole-genome sequencing. These clades exhibit distinct phylogeographic patterns, with P1 strains predominant in Asia and Africa and P2 linked to South America and West Africa, supporting an Old World origin for VARV around 3,000–4,000 years ago from a rodent-associated progenitor akin to CPXV-like viruses. Single-nucleotide polymorphism (SNP) reconstructions further indicate that VARV evolved through genome reduction, losing genes for host-range expansion present in ancestral orthopoxviruses, which enhanced human-specific adaptation but reduced zoonotic potential. Evolutionary proximity among orthopoxviruses is underscored by shared core genome elements, with VARV clustering closely with camelpox and taterapox viruses in basal phylogenetic trees, suggesting a rapid radiation from a proto-orthopoxvirus ancestor retaining all modern gene complements. Unlike broader poxvirus genera, orthopoxviruses display conserved antigenic determinants in surface proteins like hemagglutinin, enabling Bayesian phylodynamic models to trace cross-species transmission events, though VARV's strict human tropism marks its terminal evolution. Cross-immunity insights derive from antigenic similarities across es, where exposure to one species induces partial protection against others via shared epitopes on proteins. Historical observations, such as Jenner's 1796 demonstration that CPXV infection in milkmaids conferred resistance to VARV challenge, exploited this relationship, as VACV—derived from or closely related to CPXV—elicits neutralizing antibodies cross-reactive to VARV antigens. Modern serological studies confirm this, showing VACV-based vaccines generate antibodies that neutralize MPXV and VARV surrogates, with hemagglutinin sequence homology (around 85–90%) driving T-cell and B-cell , though efficacy wanes over decades post-vaccination. These findings informed smallpox eradication by leveraging VACV's broader against VARV, revealing evolutionary trade-offs: while VARV's host restriction minimized natural cross-species reservoirs, it amplified reliance on human-to-human transmission amenable to vaccine-induced . Experimental challenges in animal models further validate that prior orthopoxvirus immunity reduces VARV-like pathology, highlighting conserved immune evasion genes as targets for cross-protection.

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