Poxviridae

Poxviridae is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 22 genera that are assigned to two subfamilies: Chordopoxvirinae and Entomopoxvirinae. Entomopoxvirinae infect insects and Chordopoxvirinae infect vertebrates. Diseases associated with this family include smallpox.

Four genera of poxviruses can infect humans: Orthopoxvirus, Parapoxvirus, Yatapoxvirus, Molluscipoxvirus. Orthopoxvirus: smallpox virus (variola), vaccinia virus, cowpox virus, Mpox virus; Parapoxvirus: orf virus, pseudocowpox, bovine papular stomatitis virus; Yatapoxvirus: tanapox virus, yaba monkey tumor virus; Molluscipoxvirus: molluscum contagiosum virus (MCV). The most common are vaccinia (seen on the Indian subcontinent)^{[citation needed]} and molluscum contagiosum, but Mpox infections are rising (seen in west and central African rainforest countries). The similarly named disease chickenpox is not a true poxvirus and is caused by the herpesvirus, varicella zoster. Parapoxvirus and orthopoxvirus genera are zoonotic.

The name of the family, Poxviridae, is a legacy of the original grouping of viruses associated with diseases that produced poxes on the skin. Modern viral classification is based on phenotypic characteristics; morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The smallpox virus remains the most notable member of the family.^{[citation needed]}

Diseases caused by pox viruses, especially smallpox, have been known about for centuries. One of the earliest suspected cases is that of Egyptian pharaoh Ramses V who is thought to have died from smallpox circa 1150 years BCE. Smallpox was thought to have been transferred to Europe around the early 8th century and then to the Americas in the early 16th century, resulting in the deaths of 3.2 million Aztecs within two years of introduction. This death toll can be attributed to the indigenous population's complete lack of exposure to the virus over millennia.^{[citation needed]}

A century after Edward Jenner showed that the less potent cowpox could be used to effectively vaccinate against the more deadly smallpox, a worldwide effort to vaccinate everyone against smallpox began with the ultimate goal to rid the world of the plague-like epidemic.^{[citation needed]} The last case of endemic smallpox occurred in Somalia in 1977. Extensive searches over two years detected no further cases, and in 1979 the World Health Organization (WHO) declared the disease officially eradicated.^{[citation needed]}

In 1986, all virus samples were destroyed or transferred to two approved WHO reference labs: at the headquarters of the federal Centers for Disease Control and Prevention (the C.D.C.) in Atlanta, Georgia (the United States) and at the Institute of Virus Preparations in Moscow. After the September 11 attacks in 2001, the American and UK governments have had increased concern over the use of smallpox, or a smallpox-like disease, in bioterrorism. However, several poxviruses including vaccinia virus, myxoma virus, tanapox virus and raccoon pox virus are currently being investigated for their therapeutic potential in various human cancers in preclinical and clinical studies.

Poxviridae viral particles (virions) are generally enveloped (external enveloped virion), though the intracellular mature virion form of the virus, which contains different envelope, is also infectious. They vary in their shape depending upon the species but are generally shaped like a brick or as an oval form similar to a rounded brick because they are wrapped by the endoplasmic reticulum. The virion is exceptionally large, its size is around 200 nm in diameter and 300 nm in length and carries its genome in a single, linear, double-stranded segment of DNA. By comparison, rhinoviruses are 1/10 as large as a typical Poxviridae virion. On the outer surface membrane it has randomly arranged tubules.

Phylogenetic analysis of 26 different chordopoxvirus genomes has shown that the central region of the genome is conserved and contains ~90 genes. The termini in contrast are not conserved between species. Of this group Avipoxvirus is the most divergent. The next most divergent is Molluscipoxvirus. Capripoxvirus, Leporipoxvirus, Suipoxvirus and Yatapoxvirus genera cluster together: Capripoxvirus and Suipoxvirus share a common ancestor and are distinct from the genus Orthopoxvirus. Within the Othopoxvirus genus Cowpox virus strain Brighton Red, Ectromelia virus and Mpox virus do not group closely with any other member. Variola virus and Camelpox virus form a subgroup. Vaccinia virus is most closely related to CPV-GRI-90.^{[citation needed]}