Infectious causes of Cancer: Bacteria, Virus and Parasites

Is cancer a virus or a bacterium? No. Cancer is not an infection you can catch. Cancer is the uncontrolled growth of the body's own cells. However, some infections can raise the risk of certain cancers by damaging cells over time. So the correct picture is this: cancer itself is not a germ, but certain viruses, bacteria, and parasites can trigger the changes that lead to cancer.

Infectious agents are responsible for roughly 15 to 20 percent of cancers worldwide, a share that is higher in low- and middle-income countries. This makes them one of the most important, and most preventable, causes of cancer. Some, like the bacterium Helicobacter pylori and the human papillomavirus (HPV), account for a large share of cancers globally.

This article lists the viral, bacterial, and parasitic agents linked to cancer, explains briefly how they cause it, and summarizes them in one table.

Figure: Viruses, bacteria, and parasites linked to human cancers.

How do infections cause cancer?

An infection almost never turns into cancer quickly or in most people. Cancer develops in only a minority of infected people, usually after many years of chronic infection. Infectious agents raise cancer risk in three main ways:

• Direct genetic damage. Some viruses insert their own genes into the host cell or produce proteins that switch off the cell's natural brakes on growth. When the genes that normally control cell division or repair damage are disabled, cells can start to grow uncontrollably. HPV and its E6 and E7 proteins, which block key tumor-suppressor proteins, are the classic example.
• Chronic inflammation. Long-term infection keeps a tissue inflamed for years. Constant cycles of cell damage and repair increase the chance of a cancer-causing mutation. This is how Helicobacter pylori in the stomach and the liver flukes in the bile ducts drive cancer, and it contributes to liver cancer from hepatitis B and C.
• Weakening the immune system. Some infections suppress immunity, which lets other cancer-causing viruses go unchecked. HIV works this way. It does not cause cancer directly, but by weakening immune defenses it allows viruses like HHV-8 and EBV to trigger cancers they otherwise could not.

The key idea: the infection is usually the first step in a long chain, not an instant cause. This is also why prevention works. Removing the infection (or preventing it with a vaccine) can prevent the cancer.

Infectious agents linked to cancer

Viruses that cause cancer

Seven viruses are consistently recognized as causes of human cancer: HPV, hepatitis B and C, EBV, HHV-8, HTLV-1, and Merkel cell polyomavirus. Together, oncogenic viruses cause a large share of infection-related cancers, with HPV alone responsible for nearly half of virus-linked cancer cases.

1. Hepatitis B virus (HBV) and hepatitis C virus (HCV) are associated with liver cancer (hepatocellular carcinoma)
2. Epstein-Barr virus (EBV) is associated with lymphoma and nasopharyngeal cancer.  EBV is the primary cause of infectious mononucleosis but EBV is also a causative agent in many aggressive neoplasms, including 20% of cases of Hodgkin’s lymphoma (one of the most common lymphomas of adolescence), nasopharyngeal carcinoma, and a subtype of Burkitt’s lymphoma.
3. Human papillomavirus (HPV) is a major cause of cervical, anal, penile, and oropharyngeal cancer.  The most common clinically significant manifestation of persistent genital HPV infection is cervical intraepithelial neoplasia (CIN). HPV-16 and HPV-18 are implicated as the cause of nearly 70% of Carcinoma of the cervix. See more: Human Papillomavirus (HPV): Warts, Cervical Cancer, and the Vaccine
4. Human herpesvirus 8 (HHV-8) is the causative agent of Kaposi’s sarcoma.
5. Human immunodeficiency virus (HIV): HIV does not cause cancer directly but HIV infection increases a person’s risk of getting several types of cancer such as Kaposi’s sarcoma, invasive cervical cancer, non-Hodgkin lymphoma, and central nervous system lymphoma, etc.
6. Human T-lymphotropic virus-1 (HTLV-1): HTLV-1 has been linked with a type of lymphocytic leukemia and non-Hodgkin lymphoma called adult T-cell leukemia/lymphoma (ATL).
7. Merkel cell polyomavirus (MCPyV) is the cause of most cases of Merkel cell carcinoma, an aggressive skin cancer. It was one of the more recently discovered human cancer viruses.

Figure: Various cancers have been linked to infection

Bacteria that cause cancer

Only one bacterium is a well-established cause of human cancer, but it is a major one. Chronic infection with Helicobacter pylori is the single largest infectious cause of cancer worldwide by number of cases. It predisposes to stomach cancer and to gastric mucosa-associated lymphoid tissue (MALT) lymphoma, by keeping the stomach lining inflamed for years. Treating the infection can reduce this risk, which is why H. pylori is considered an important preventable cause of cancer.

Parasites that cause cancer

Three parasitic flatworms (flukes) are recognized causes of cancer, mainly in the regions where they are common:

• Schistosoma haematobium is linked to bladder cancer. Its eggs lodge in the bladder wall and cause long-term inflammation. It is common in parts of Africa and the Middle East.
• Opisthorchis viverrini and Clonorchis sinensis are liver flukes linked to cancer of the bile ducts (cholangiocarcinoma). They are acquired from eating raw or undercooked freshwater fish and are common in parts of Southeast and East Asia.

How to Remember

Cancer is not a germ, but germs can start it: you cannot "catch" cancer like a cold. What you can catch is an infection that, years later, may trigger cancer. Infection is the match, not the fire.

The three routes ("break it, burn it, or blind the guard"): infections cause cancer by directly breaking the cell's growth-control genes (HPV), by burning the tissue with chronic inflammation (H. pylori, flukes, hepatitis), or by blinding the immune guard so other viruses run wild (HIV).

The two biggest culprits ("a bug and a wart virus"): by case numbers, the two largest infectious causes of cancer worldwide are a bacterium (Helicobacter pylori, stomach cancer) and a virus (HPV, cervical and other cancers). Both are preventable, one by treatment, one by vaccine.

Flukes and their targets ("blood fluke to bladder, liver fluke to bile duct"): Schistosoma haematobium (a blood fluke) goes to the bladder; Opisthorchis and Clonorchis (liver flukes) go to the bile ducts. Match the worm's home to the cancer's site.

Key exam facts

Where Students Get Confused

"Is cancer a virus or a bacterium?" Neither. Cancer is the uncontrolled growth of a person's own cells, not a germ. You cannot catch cancer from someone. What can be caught is an infection that, over years, raises the risk of certain cancers. This is the single most common misunderstanding about cancer and infection.

"If I get HPV or hepatitis B, will I get cancer?" Almost certainly not. Cancer develops in only a small minority of infected people, and usually only after many years of persistent infection. Most HPV infections, for example, clear on their own. The infection raises risk; it does not guarantee cancer.

"Is HPV a virus, a bacterium, or a parasite?" HPV is a virus (human papillomavirus). This confusion is common because it is grouped with bacteria and parasites as an "infectious cause of cancer," but the three are completely different kinds of organism.

"Does HIV cause cancer?" Not directly. HIV weakens the immune system, which then allows other cancer-causing viruses (like HHV-8 and EBV) to trigger cancers the body would normally control. The cancer risk from HIV is real but indirect.

"How can a stomach bug cause cancer?" Helicobacter pylori lives in the stomach and causes long-term inflammation of its lining. Years of this inflammation, with repeated cell damage and repair, increase the chance of a cancer-causing mutation. It is the leading infectious cause of cancer worldwide by case count.

"Can these cancers be prevented?" Often, yes, which is what makes infectious causes so important. Vaccines prevent HPV and hepatitis B infection (and therefore the cancers they cause), and treating H. pylori or hepatitis C can lower risk. Prevention of the infection is prevention of the cancer.

References

1. de Martel, C., Georges, D., Bray, F., Ferlay, J., & Clifford, G. M. (2020). Global burden of cancers attributable to infections in 2018: a worldwide incidence analysis. The Lancet Global Health, 8(2), e180–e190. https://doi.org/10.1016/S2214-109X(19)30488-7
2. American Cancer Society. (2024). Viruses that can lead to cancer. https://www.cancer.org/cancer/cancer-causes/infectious-agents/infections-that-can-lead-to-cancer/viruses.html
3. American Cancer Society. (2024). Bacteria that can lead to cancer. https://www.cancer.org/cancer/cancer-causes/infectious-agents/infections-that-can-lead-to-cancer/bacteria.html
4. American Cancer Society. (2024). Parasites that can lead to cancer. https://www.cancer.org/cancer/cancer-causes/infectious-agents/infections-that-can-lead-to-cancer/parasites.html
5. Krump, N. A., & You, J. (2018). Molecular mechanisms of viral oncogenesis in humans. Nature Reviews Microbiology, 16(11), 684–698. https://doi.org/10.1038/s41579-018-0064-6