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Cancer is a group of diseases defined by abnormal cell growth and caused by genetic defects.  Most of us know that these genetic defects can be inherited, caused by environmental factors such as smoking, drinking, obesity and a lack of physical activity, as well as exposure to radiation and pollution.  It is not commonly known, however, that many infectious agents, including some viruses (oncoviruses), bacterial infections, and parasites, can increase one’s probability of developing cancer.

The modes by which these agents can either cause cancer, or inhibit the body’s ability to fight cancer, vary.  Some bacterial and viral infections, as well as parasites, can cause chronic inflammation, which can then lead to cancer.  Some viruses can disrupt biochemical signaling, which controls the rate of cell growth.  Other infections, such as HIV, can weaken the body’s immune system, and therein, its own innate ability to fight cancer. 

Some of the infectious agents tied to cancer include the Hepatitis B Virus and Hepatitis C Virus (HBV and HCV), Human Immunodeficiency Virus (HIV), Human Papillomaviruses (HPVs), Epstein-Barr Virus (EBV), Human T-Cell Leukemia/Lymphoma Virus Type 1 (HTLV-1), Kaposi Sarcoma-Associated Herpesvirus (KSHV), Kaposi Sarcoma-Associated Herpesvirus (KSHV), Opisthorchis viverrini, and Helicobacter pylori (H. pylori).

Epstein-Barr Virus (EBV) has been tied to some forms of fast-growing lymphomas such as Burkitt lymphoma as well as nasopharyngeal cancer (cancer of the area in the back of the nose).   EBV is most commonly transmitted via saliva; by the teenage years, a large majority of the population (90%) will become infected with EBV. 

Human Papillomaviruses (HPVs) is a sexually transmitted disease, one of the most common STDs, and has been identified as the one of the main causes of cervical cancer.  Nearly all women with cervical cancer, the second most common form of cancer in women, test HPV positive.  Pap tests have been effective at lowering the incidence of cervical cancer, by allowing doctors to identify and remove pre-cancerous cells caused by HPV.

Human immunodeficiency virus (HIV), which is most commonly spread via sexual intercourse, blood, and breast milk, increases the risk of several types of cancer, including Kaposi sarcoma, cervical cancer, anal cancer, lung cancer, and certain non-Hodgkin lymphomas.  The HIV virus destroys T-cells, weakening the body’s own immune system, and its ability to kill new cancer cells before they evolve into serious, life-threatening tumors.

Both Hepatitis B virus (HBV) and Hepatitis C virus (HCV) have been tied to increased incidence of liver cancer.  Nearly half of all liver cancers, worldwide, have been tied to HBV and HCV.  HBV and HCV are spread via blood (needles and transfusions), sex, and childbirth.

Certain forms of the Herpes virus, most notably HHV-8, also known as Kaposi sarcoma–associated herpes virus (KSHV), have been found in the overwhelming majority of patents with Kaposi sarcoma, a slow-growing form of skin cancer.  HHV-8 is spread through sex, as well as blood and saliva.

Human T-Cell Leukemia/Lymphoma Virus Type 1 (HTLV-1) is a retrovirus.  Retrovirus are a class of viruses that, though the unusual way they replicate, can alter the DNA of the human cells they infect, thereby leading to mutations and increased rates of cancer.  HTLV-1 has been linked to an increased incidence of lymphocytic leukemia and non-Hodgkin lymphoma.

The Merkel cell polyomavirus (MCV) causes a rare form of skin cancer called Merkel cell carcinoma that usually appears as flesh-colored or bluish nodules on the neck or face – 80% of Merkel cell carcinoma (MCC) tumors ae infected with MCV.

Opisthorchis viverrini and Clonorchis sinensis are flatworms that have been linked to an increased risk of developing cancer of the bile ducts; they are contracted by eating undercooked fish.  Schistosoma hematobium is another flatworm found in contaminated fresh water, which can burrow into the skin and has been linked to bladder cancer.

Helicobacter pylori, or H. pylori, is a bacterium that infects the mucus layer coating the inside of the stomach, and is typically spread via contaminated water, food, and kissing.  A 2001 meta-analysis of 12 case controlled studies concluded that individuals infected with H. pylori were 6 times more likely to contract non-cardia gastric cancer than those not infected by the bacteria.

When considering a lifestyle that reduces cancer risk, we should focus not only on traditional environmental factors such as food, smoking, and exposure to sun, but also controllable elements such as contact with infected individuals and contaminated foods and water, as well as safe sexual practices.