Although most cancer seems to arise from environmental exposures to chemicals and radiation or lifestyle choices, a large number of cases are caused by infections. As more is learned about different types of cancer, additional links with infectious agents are likely to be found.
Parasites
One of the first connections between infections and cancer was provided by Dr. Johannes Fibiger. Dr. Fibiger noticed an association between cancer in his laboratory rats and infection with a parasitic worm (nematode). He named the worm Spiroptera carcinoma. While it was later shown that the worms were not the primary cause of the cancer, the link between infection and cancer has withstood the test of time.(1)
Bacteria
Helicobacter pylori is a bacteria that is able to survive in the mucosa of the gastric (stomach) epithelium for a long time and is a main cause of stomach and duodenal diseases. Epidemiological studies have provided evidence that H. pylori is associated with the development of lymphoma in mucosa-associated lymphoid tissue (MALT) and gastric adenocarcinoma (stomach cancer). (2)
Recent research suggests that the populations of bacteria that live in a person's instestines can influence their risk of developing colon cancer. XXX
Viruses
Certain viruses and bacteria have also been associated with the initiation and promotion of tumor growth based on both epidemiological and experimental evidence. Some DNA viruses contain genes whose products can take control of cell division in the host cell. They promote the development of tumors by increasing proliferation rates and shutting down the normal systems that prevent cells from dividing. These viruses include human papillomavirus (HPV), the most significant risk factor for the development of cervical cancer. Similarly, an RNA virus, human T-cell leukemia virus type 1 (HTLV-1) leads to adult T-cell leukemia by stimulating the proliferation of infected T-cells. (3) Epstein-Barr virus (EBV) is another virus that increases cell proliferation. This virus also protects infected cells from death (apoptosis). EBV infects more than 90% of the world's adult population and has been implicated as a cause of Burkitt's lymphoma, nasopharyngeal carcinoma and gastric lymphoma. Experimental evidence in rats and from human tumors, has implicated the JC virus (a polyomavirus)in brain tumors, particularly medulloblastomas. Some viruses also have indirect actions on tumor development. Hepatitis B causes damage that leads to increased cell division and inflammation in the liver, potentially promoting the growth of tumors. The human immunodeficiency virus (HIV) greatly reduces the function of the immune system and is the cause of AIDS. HIV infection may also make patients susceptible to infection by a type of human herpes virus 8 (HHV8), a risk factor for Kaposi's sarcoma.
Learn more about viruses and cancer in the next section.
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