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Infectious causes of cancer
Estimates place the worldwide risk of cancers from infectious causes at 16.1%. Viral infections are risk factors for cervical cancer, 80% of liver cancers, and 15–20% of the other cancers. This proportion varies in different regions of the world from a high of 32.7% in Sub-Saharan Africa to 3.3% in Australia and New Zealand.
A virus that can cause cancer is called an oncovirus or tumor virus. These include the human papillomavirus, which is associated with cervical carcinoma and nasopharyngeal carcinoma; Epstein-Barr virus, which is associated with a variety of Epstein-Barr virus-associated lymphoproliferative lymphomas; Kaposi's sarcoma herpesvirus, which is associated with Kaposi's sarcoma and primary effusion lymphoma; hepatitis B and hepatitis C viruses which are associated with hepatocellular carcinoma; and human T-cell leukemia virus-1 which is associated with adult T-cell leukemia/lymphoma. Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced stomach cancer. Parasitic infections strongly associated with cancer include Schistosoma haematobium (squamous cell carcinoma of the bladder) and the liver flukes, Opisthorchis viverrini and Clonorchis sinensis (cholangiocarcinoma).
Infection is the fourth most important risk factor for cancer mortality in the developed world, causing about 10% of cancer mortality (see cancer prevention), coming after tobacco (~30% of cancers), diet (~30%) and obesity (~15%). Cancer causes 22.5% of deaths in the United States, so that about 2% of mortality in the United States appears to be due to cancers caused by infections. This is comparable to mortality caused by influenza and pneumonia, which cause 2.1% of deaths in the United States.
Worldwide in 2015, the most common causes of cancer death were lung cancer (1.6 million deaths), liver cancer (745,000 deaths), and stomach cancer (723,000 deaths). Lung cancer is largely due to non-infectious causes, such as tobacco smoke. However, liver and stomach cancer are primarily due to infectious causes. Liver cancer is largely caused by infectious hepatitis B virus (HBV) plus hepatitis C virus (HBC) and stomach cancer is largely caused by Helicobacter pylori bacteria. World-wide, the estimated number of people chronically infected with HBV and/or HCV is ~325 million. Over half of the world's population is colonized with H. pylori and it is estimated that H. pylori-positive patients have a 1-2% risk of developing distal gastric cancer.
Genomic instability through various means such as DNA damage and epigenetic modifications appear to be the basic causes of sporadic (non-familial) cancer. While infections have many effects, infectious organisms that increase the risk of cancer are frequently a source of DNA damage or genomic instability, as discussed below for oncogenic viruses and an oncogenic bacterium.[citation needed]
Viruses are one of the most important risk factors for cancer development in humans.
Infection by some hepatitis viruses, especially hepatitis B and hepatitis C, can induce a long-term viral infection that leads to liver cancer in about 1 in 200 of people infected with hepatitis B each year (more in Asia, fewer in North America), and in about 1 in 45 of people infected with hepatitis C each year. People with chronic hepatitis B infection are more than 200 times more likely to develop liver cancer than uninfected people. Liver cirrhosis, whether from chronic viral hepatitis infection or excessive alcohol use or some other cause, is independently associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. Because chronic viral hepatitis is so common, and liver cancer so deadly, liver cancer is one of the most common causes of cancer-related deaths in the world, and is especially common in East Asia and parts of sub-Saharan Africa.[citation needed]
Human papillomaviruses (HPV) also cause many cancers. HPV is well known for causing genital warts and essentially all cases of cervical cancer, but it can also infect and cause cancer in several other parts of the body, including the esophagus larynx, lining of the mouth, nose, and throat, anus, vulva, vagina, and penis. The Papanicolaou smear ("Pap" smear) is a widely used cancer screening test for cervical cancer. DNA-based tests to identify the virus are also available.
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Infectious causes of cancer
Estimates place the worldwide risk of cancers from infectious causes at 16.1%. Viral infections are risk factors for cervical cancer, 80% of liver cancers, and 15–20% of the other cancers. This proportion varies in different regions of the world from a high of 32.7% in Sub-Saharan Africa to 3.3% in Australia and New Zealand.
A virus that can cause cancer is called an oncovirus or tumor virus. These include the human papillomavirus, which is associated with cervical carcinoma and nasopharyngeal carcinoma; Epstein-Barr virus, which is associated with a variety of Epstein-Barr virus-associated lymphoproliferative lymphomas; Kaposi's sarcoma herpesvirus, which is associated with Kaposi's sarcoma and primary effusion lymphoma; hepatitis B and hepatitis C viruses which are associated with hepatocellular carcinoma; and human T-cell leukemia virus-1 which is associated with adult T-cell leukemia/lymphoma. Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced stomach cancer. Parasitic infections strongly associated with cancer include Schistosoma haematobium (squamous cell carcinoma of the bladder) and the liver flukes, Opisthorchis viverrini and Clonorchis sinensis (cholangiocarcinoma).
Infection is the fourth most important risk factor for cancer mortality in the developed world, causing about 10% of cancer mortality (see cancer prevention), coming after tobacco (~30% of cancers), diet (~30%) and obesity (~15%). Cancer causes 22.5% of deaths in the United States, so that about 2% of mortality in the United States appears to be due to cancers caused by infections. This is comparable to mortality caused by influenza and pneumonia, which cause 2.1% of deaths in the United States.
Worldwide in 2015, the most common causes of cancer death were lung cancer (1.6 million deaths), liver cancer (745,000 deaths), and stomach cancer (723,000 deaths). Lung cancer is largely due to non-infectious causes, such as tobacco smoke. However, liver and stomach cancer are primarily due to infectious causes. Liver cancer is largely caused by infectious hepatitis B virus (HBV) plus hepatitis C virus (HBC) and stomach cancer is largely caused by Helicobacter pylori bacteria. World-wide, the estimated number of people chronically infected with HBV and/or HCV is ~325 million. Over half of the world's population is colonized with H. pylori and it is estimated that H. pylori-positive patients have a 1-2% risk of developing distal gastric cancer.
Genomic instability through various means such as DNA damage and epigenetic modifications appear to be the basic causes of sporadic (non-familial) cancer. While infections have many effects, infectious organisms that increase the risk of cancer are frequently a source of DNA damage or genomic instability, as discussed below for oncogenic viruses and an oncogenic bacterium.[citation needed]
Viruses are one of the most important risk factors for cancer development in humans.
Infection by some hepatitis viruses, especially hepatitis B and hepatitis C, can induce a long-term viral infection that leads to liver cancer in about 1 in 200 of people infected with hepatitis B each year (more in Asia, fewer in North America), and in about 1 in 45 of people infected with hepatitis C each year. People with chronic hepatitis B infection are more than 200 times more likely to develop liver cancer than uninfected people. Liver cirrhosis, whether from chronic viral hepatitis infection or excessive alcohol use or some other cause, is independently associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. Because chronic viral hepatitis is so common, and liver cancer so deadly, liver cancer is one of the most common causes of cancer-related deaths in the world, and is especially common in East Asia and parts of sub-Saharan Africa.[citation needed]
Human papillomaviruses (HPV) also cause many cancers. HPV is well known for causing genital warts and essentially all cases of cervical cancer, but it can also infect and cause cancer in several other parts of the body, including the esophagus larynx, lining of the mouth, nose, and throat, anus, vulva, vagina, and penis. The Papanicolaou smear ("Pap" smear) is a widely used cancer screening test for cervical cancer. DNA-based tests to identify the virus are also available.