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.
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 wormSpiroptera 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)
Several different kinds of worm-like parasites (trematodes) are known to cause cancer. The parasites are usually very small and have complicated life cycles, living in several different hosts at different stages in their lives. The life cycle of Clonorchis sinensis is shown above.
The trematodes known to cause cancer include:
- Schistosoma haematobium (above left) causes bladder cancer. The image is a highly magified view (electron micrograph) of a schistosome.
- Clonorchis sinensis (above center) and Opisthorchis viverrini (above right): these two ‘flukes’ infect the bile duct and are associated with cancer of the bile duct (cholangiocarcinoma) and liver (hepatocarcinoma).(2)(3)
Helicobacter pylori (pictured below) 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). (4)
Recent research suggests that the populations of bacteria that live in a person’s instestines/colon can increase or decrease their of developing colon cancer.(5)(6)(7)
Some viruses 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. HPV also causes cancers of the head/neck, anal cancer, and urogenital cancers.(8)(9)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. (10)(11)(12) 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.(13)(14) The image below is of leukemia cells infected with EBV. Some viruses also have indirect effects on tumor development. Hepatitis B causes damage that leads to increased cell division and inflammation in the liver, potentially promoting the growth of tumors.(15) 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, HHV 8, that is a risk factor for Kaposi’s sarcoma(16). Much more information about viruses that cause cancer.