Certain infectious agents, including viruses, bacteria, and parasites, can cause cancer or increase the risk that cancer will form. Some viruses can disrupt signaling that normally keeps cell growth and proliferation in check. Also, some infections weaken the immune system, making the body less able to fight off other cancer-causing infections. And some viruses, bacteria, and parasites also cause chronic inflammation, which may lead to cancer.

Most of the viruses that are linked to an increased risk of cancer can be passed from one person to another through blood and/or other body fluids. As described below, you can lower your risk of infection by getting vaccinated, not having unprotected sex, and not sharing needles.

EBV, a type of herpes virus, causes mononucleosis as well as certain types of lymphoma and cancers of the nose and throat. EBV is most commonly transmitted by contact with saliva, such as through kissing or by sharing toothbrushes or drinking glasses. It can also be spread by sexual contact, blood transfusions, and organ transplantation. EBV infection is lifelong. More than 90% of people worldwide will be infected with EBV during their lifetime, and most do not develop any symptoms. There is no vaccine to prevent EBV infection and no specific treatment for EBV infection.

Chronic infections with HBV or HCV can cause liver cancer. Both viruses can be transmitted via blood (for example, by sharing needles or through blood transfusions) and from mother to baby at birth. In addition, HBV can be transmitted via sexual contact.

Since the 1980s, infants in the United States and most other countries have been routinely vaccinated against HBV infection. Experts recommend that adults who have not been vaccinated against HBV and are at increased risk of HBV infection get vaccinated as soon as possible. Vaccination is especially important for health care workers and other professionals who come into contact with human blood.

The U.S. Centers for Disease Control and Prevention (CDC) also recommends that everyone in the United States born from 1945 through 1965, and other populations at increased risk for HCV infection, be tested for HCV. Although there is not currently a vaccine against HCV, new therapies can cure people of HCV infection. If you think you may be at risk for HBV or HCV infection, ask your doctor about being tested. These infections do not always cause symptoms, but tests can show whether you have the virus. If so, your doctor may suggest treatment. Also, your doctor can tell you how to keep from infecting other people.

HIV is the virus that causes acquired immunodeficiency syndrome (AIDS). HIV does not cause cancer itself, but infection with HIV weakens the immune system and makes the body less able to fight off other infections that cause cancer. People infected with HIV have increased risks of a number of cancers, especially Kaposi sarcoma, lymphomas (including both non-Hodgkin lymphoma and Hodgkin disease), and cancers of the cervix, anus, lung, liver, and throat.

HIV can be transmitted via blood and through sexual contact. Men who have unprotected sex with other men and people who share needles for injection drug use are at the highest risk of acquiring HIV infection; heterosexual individuals who have unprotected sex with multiple partners are at the next highest risk.

People can be infected with HIV for years before they begin to develop symptoms. If you think you may be at risk for HIV infection, ask your doctor about being tested. If you test positive, your doctor can prescribe highly effective antiviral treatment and can tell you how to keep from infecting other people.

Infection with high-risk types of HPV cause nearly all cervical cancers. They also cause most anal cancers and many oropharyngeal, vaginal, vulvar, and penile cancers. High-risk HPVs spread easily through direct sexual contact, including vaginal, oral, and anal sex. Several vaccines have been developed that prevent infection with the types of HPV that cause most HPV-associated cancers. In the United States, experts recommend that children be vaccinated at age 11 or 12, but children as young as age 9 and adults as old as 26 can also be vaccinated.

Cervical cancer screening can be used to detect signs of HPV infections in the cervix. Although HPV infections themselves cannot be treated, the cervical abnormalities that these infections can cause over time can be treated.

HTLV-1 can cause an aggressive type of non-Hodgkin lymphoma called adult T-cell leukemia/lymphoma (ATLL). This virus spreads via blood (by sharing needles or through transfusions), through sexual contact, and from mother to child in the womb or via breastfeeding. Infection with this virus is more common in Japan, Africa, the Caribbean, and South America than in the United States. Most people with HTLV-1 infection do not have any symptoms or develop disease.

Blood is routinely screened for HTLV-1 in the United States. There is no vaccine to prevent infection with this virus and no treatment if you are infected. If you think you may be at risk for HTLV-1 infection, ask your doctor about being tested. If you test positive, your doctor can tell you how to keep from infecting other people and monitor you for HTLV-1-induced disease.

Kaposi sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus-8 (HHV-8), can cause Kaposi sarcoma. KSHV can also cause primary effusion lymphoma and multicentric Castleman disease.

KSHV is most commonly spread through saliva. It can also be spread through organ or bone marrow transplantation, and there is some evidence that it can be spread by blood transfusion, although this risk is minimized by practices followed in the United States such as blood storage and removal of white cells.

KSHV infection is generally limited to certain populations, and the way KSHV is spread varies among these populations. In sub-Saharan Africa and certain regions of Central and South America, where KSHV infection is relatively common, it is believed to spread by contact with saliva among family members. In Mediterranean countries (Italy, Greece, Israel, Saudi Arabia), where KSHV infection is present at intermediate levels, it is thought to spread by contact among children and by ill-defined routes among adults. Finally, in regions where KSHV infection is uncommon, such as the United States and Northern Europe, it appears to be mostly transmitted sexually, especially among men who have sex with men.

Most people infected with KSHV do not develop cancer or show any symptoms, although those who also have HIV infection or are immunosuppressed for other reasons are more likely to develop KSHV-caused diseases. There is no vaccine to prevent KSHV infection and no therapy to treat infection. Men who have sex with men may be advised to avoid oral–anal contact (including use of saliva as a personal lubricant). And people who are infected with HIV can lower their risk of KSHV-related complications by using antiretroviral therapy.

MCPyV can cause Merkel cell carcinoma, a rare type of skin cancer. Most adults are infected with MCPyV, with transmission most likely occurring through casual direct (i.e., skin-to-skin) or indirect (i.e., touching a surface that an infected person has touched) contact in early childhood. The risk of Merkel cell carcinoma is greatly increased in elderly people and in younger adults with who are infected with HIV or are immunosuppressed for other reasons. Infection does not generally cause symptoms, and there are no treatments for MCPyV.

H. pylori is a type of bacterium that can cause noncardia gastric cancer (a type of stomach cancer) and a type of lymphoma in the stomach lining, gastric MALT lymphoma. It can also cause stomach ulcers. The bacterium is thought to spread through consumption of contaminated food or water and direct mouth-to-mouth contact. The CDC estimates that approximately two-thirds of the world’s population harbors H. pylori, with infection rates much higher in developing countries than in developed nations. In most populations, the bacterium is first acquired during childhood.

If you have stomach problems, see a doctor. Infection with H. pylori can be detected and treated with antibiotics.

Having colitis or certain liver diseases can increase the risk of bile duct cancer.

Anything that increases your risk of getting a disease is called a risk factor. Having a risk factor does not mean that you will get cancer; not having risk factors doesn't mean that you will not get cancer. People who think they may be at risk should discuss this with their doctor.

Risk factors for bile duct cancer include the following conditions:

• Primary sclerosing cholangitis (a progressive disease in which the bile ducts become blocked by inflammation and scarring).
• Chronic ulcerative colitis.
• Cysts in the bile ducts (cysts block the flow of bile and can cause swollen bile ducts, inflammation, and infection).
• Infection with a Chinese liver fluke parasite.

This parasitic flatworm (fluke), which lives in certain types of freshwater snails found in Africa and the Middle East, can cause bladder cancer. People become infected when infectious free-swimming flatworm larvae burrow into skin that has come into contact with contaminated fresh water. Antiparasitic drugs are used to treat the infection.

XMRV is a newly identified human retrovirus that is similar to a group of mouse retroviruses (called murine leukemia viruses, or MLVs) scientists have known about for years. XMRV refers to xenotropic murine leukemia virus-related virus. It was first identified in 2006 in tissue samples from men with prostate cancer.

In a study published in the journal Science in October 2009, scientists reported a potential association of XMRV with chronic fatigue syndrome (CFS). In this study, XMRV was detected in approximately two-thirds of patients diagnosed with CFS. They also identified DNA of XMRV in the blood cells of some healthy persons and suggested a potential for XMRV transmission by transfusion or transplantation.

However, other recent studies, including a July 2010 research report from CDC scientists and colleagues at two other institutions, found no evidence of XMRV in CFS patients and in controls (see Updates).

More recently, investigators from the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and Harvard Medical School published a report that presents evidence of MLVs in blood samples from CFS patients and healthy blood donors. The authors state that although they found a broader group of MLVs, rather than XMRV, their results support the 2009 report in Science. The FDA /NIH paper was published online August 23, 2010, in the Proceedings of the National Academy of Sciences.

The reporting of different findings from different studies is not uncommon. Various factors may have contributed to the differences in these studies, including selection criteria for inclusion of CFS patients, clinical complexities of CFS, and possible variations in XMRV and MLV infection rates among populations in different regions. Moreover, XMRV is a recently discovered virus and much remains to be learned about this and MLV-like viruses. As additional studies are done, it is possible that new findings may emerge that differ from what has been previously reported.

The potential role of XMRV and MLVs in causing diseases such as prostate cancer and CFS remains unknown at this time. Additional research is needed to further evaluate a possible link of XMRV and MLVs with negative health outcomes, including prostate cancer and CFS. If it is determined that XMRV and MLVs may have a role in causing disease and illness, prevention recommendations can be made.

Although it is presumed that XMRV can be transmitted through blood transfusion, no such transmission event has been identified, and there is no known evidence of XMRV or MLV infection or related illness or disease in transfusion recipients. Agencies within the Department of Health and Human Services (HHS) are conducting studies to determine the prevalence of XMRV in the blood donor population.

CDC, FDA, and NIH investigators have been collaborating with scientists from other agencies and groups with regard to XMRV and MLV research, and they will continue to do so. HHS agencies will keep the public updated and informed as more information becomes available.