Infectious causes of cancer
Worldwide 16.1% of cancers are related to infectious diseases. 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. Viruses are the usual infectious agents that cause cancer but Mycobacterium, some other bacteria and parasites also have an effect.
A virus that can cause cancer is called an oncovirus. These include human papillomavirus (cervical carcinoma), Epstein-Barr virus (B-cell lymphoproliferative disease and nasopharyngeal carcinoma), Kaposi's sarcoma herpesvirus (Kaposi's Sarcoma and primary effusion lymphomas), hepatitis B and hepatitis C viruses (hepatocellular carcinoma), and Human T-cell leukemia virus-1 (T-cell leukemias). Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced gastric carcinoma. 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).
There are two types of cancers caused by viruses: acutely transforming or slowly transforming cancers. In acutely transforming viruses, the virus carries an overactive oncogene, and the infected cell becomes cancerous as soon as the overactive viral gene is expressed. In contrast, in slowly transforming viruses, the virus genome is inserted near a previously existing proto-oncogene in the genome of the infected cell. The virus causes overexpression of that proto-oncogene, which typically induces uncontrolled cell division. Because the virus' genes might not insert near enough to a proto-oncogene to trigger the cancerous changes and, even if optimally located, it might take some time to become activated, slowly transforming viruses usually cause tumors much longer after infection than the acutely transforming viruses, if at all.
Infection by some hepatitis viruses, especially hepatitis B and hepatitis C, can induce a chronic 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 alcohol abuse 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-Sarahan Africa.
Human papillomaviruses (HPV) are another particularly common cancer-causing virus. 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 larynx, lining of the mouth, nose, and throat, anus, and esophagus. 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.
Herpesviruses are a third group of common cancer-causing viruses. Two types of herpesviruses have been associated with cancer: the Epstein-Barr virus (EBV) and human herpesvirus 8 (HHV-8). EBV appears to cause all nonkeratinizing nasopharyngeal carcinomas and some cases of lymphoma, including Burkitt’s lymphoma—the association is especially strong in Africa—and Hodgkin’s disease. EBV has also been found in a variety of other types of cancer cells, although its role in causing these other cancers is not well established. HHV-8 causes all cases of Kaposi’s sarcoma, and has been found in some cases of a cancer-related condition called Castleman's disease. Studies involving other kinds of cancer, particularly prostate cancer, have been inconsistent. Both of these herpesviruses are commonly found in cancerous cells of primary effusion lymphoma. Herpesviruses also cause cancer in animals, especially leukemias and lymphomas.
Still a controversial subject, the Polyomavirus SV-40 has been known to cause soft tissue cancers in nearly all mammal groups tested. It was discovered in 1960  that the polyoma virus, renamed in 1959 to Simian Virus 40, SV40 had contaminated up to 30%, and much higher percentages have been debated, of the poliovirus vaccines in the USA. The contamination unfortunately happened because the early vaccines were created in monkey kidney cell cultures riddled with SV40 and many other unknown simian viruses between 1955 and 1963. It has been estimated and calculated that during the earliest polio inoculations mainly contrived of Dr. Salk's ground up monkey kidney vaccine and Dr. Sabin's live attenuated vaccine via sugar cube, an estimated 60% of adults and 90% of children (about 98 million Americans at the time) were possibly exposed to these SV40 contaminated vaccines. This wide exposure of the US population to SV40 has understandably created many scientific and public health concerns and debates. This is because the Macaque polyomavirus has been documented to be oncogenic in rodents and also capable of transforming human cells in vitro. Sarcoma and ependymoma rapidly developed in hamsters injected with monkey cells infested with SV40. Other experiments in vitro have also supported these findings and have also determined that SV40 is very capable of the transformation of human cells.    Furthermore and very important, these previously transformed human cells were capable of creating tumor growth when injected into conscious and willing terminally ill human patients. Researchers, though not all are looking, have detected many instances of SV40 DNA sequences in several extracted rare human tumors, including ependymoma, osteosarcomas, and mesothelioma.
HIV does not directly cause cancer, but it is associated with a number of malignancies, especially Kaposi's sarcoma, non-Hodgkin's lymphoma, anal cancer and cervical cancer. Kaposi's sarcoma is caused by human herpesvirus 8. AIDS-related cases of anal cancer and cervical cancer are commonly caused by human papillomavirus. After HIV destroys the immune system, the body is no longer able to control these viruses, and the infections manifest as cancer. Certain other immune deficiency states (e.g. common variable immunodeficiency and IgA deficiency) are also associated with increased risk of malignancy.
In addition to viruses, certain kinds of bacteria can cause some cancers. The most prominent example is the link between chronic infection of the wall of the stomach with Helicobacter pylori and gastric cancer. Although only a minority of those infected with Helicobacter go on to develop cancer, since this bacterial infection is quite common, it may be responsible for most of these cancers. The mechanism by which H. pylori causes cancer may involve chronic inflammation, or the direct action of some of its virulence factors, for example, CagA has been implicated in carcinogenesis.
The parasites that cause schistosomiasis (bilharzia), especially S. haematobium, can cause bladder cancer and cancer at other sites. Inflammation triggered by the worm's eggs appears to be the mechanism by which squamous cell carcinoma of the bladder is caused. In Asia, infection by S. japonicum is associated with colorectal cancer.
Parasites are also a significant cause of cancer in animals. Cysticercus fasciolaris, the larval form of the common tapeworm of the cat, Taenia taeniaformis, causes cancer in rats. Spirocerca lupi is associated with esophageal cancer in dogs, at least within the southern United States.
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