Common infections such as those that cause the common cold, strep throat, some skin conditions, or diarrheal diseases do not cause cancer or make cancer more likely to occur. However, infections with specific types of viruses, bacteria, or parasites can increase an individual’s risk for certain kinds of cancer.
There are a variety of ways that infections can increase a person’s cancer risk. Viruses may enter cells and alter genes associated with cell growth and division; certain infections may cause long-term inflammation in a part of the body; and some infections lower the immune system’s ability to fight cancer.
While infection with certain agents can increase an individual’s cancer risk, the vast majority of people who acquire such infections do not develop cancer. In many cases, infections can be prevented or treated, lowering the risk for related cancers.
Bacterial infections and cancer
Most forms of bacteria have no connection to cancer, but a few have been linked to the disease. One of these is Helicobacter pylori, which causes ulcers and can inflame and damage the inner lining of the stomach, increasing the risk of stomach cancer and lymphoma of the stomach. While an estimated two-thirds of all adults in the world are infected by H. pylori, stomach cancer is quite rare in industrialized countries.
A variety of factors influences an individual’s risk of stomach cancer, notably the safety of the food and water supply. The Centers for Disease Control and Prevention recommends that people with ulcers or a history of ulcers be treated with antibiotics for H. pylori if they’re infected with the bacterium.
Some studies have shown that women who are or have been infected with chlamydia trachomatis may be at greater risk for cervical cancer. This common bacterium, often referred to as just “chlamydia,” can infect the female reproductive tract as well as other parts of the body in both men and women. Research suggests that chlamydia doesn’t cause cancer on its own, but interacts with human papilloma virus (HPV) in a manner that promotes cancer growth. Because infection with chlamydia often doesn’t produce symptoms, many women who carry the infection don’t realize they’re infected. If a blood test or pelvic exam reveals a chlamydia infection, it can be treated with antibiotics.
Scientists have begun to explore whether a disruption in the natural balance of bacteria in the digestive tract can contribute to cancer. In 2011, for example, Dana-Farber researchers discovered that colorectal cancer tissue contains high levels of several types of bacteria, notably Fusobacterium nucleatum, which isn’t found in the the colon. More recently, the researchers found that Fusobacterium often travels with colon cancer cells when they metastasize to other parts of the body.
It’s known that healthy diets — rich in whole grains and fiber — appear to protect against colorectal cancer. A recent study by Dana-Farber researchers suggests that healthy foods achieve this benefit, in part, by altering the relative amounts of various microorganisms in the digestive tract, including the bacterium Fusobacterium nucleatum.
Viral infections and cancer
One of the types of virus most frequently associated with cancer is HPV, a group of more than 150 related viruses. As with H. pylori, infection with HPV is very common around the world. The immune system is usually capable of clearing or controlling HPV infections and as a result, most people infected with HPV do not develop infection-related cancers.
However a few types of HPV are among the main causes of cervical cancer and can also play a role in cancers of the vagina, vulva, and penis. The virus is also associated with cancer of the anus and with certain cancers of the mouth and throat.
Vaccines can protect against the main cancer-related strains of HPV. The American Cancer Society recommends that routine HPV vaccination for girls and boys begin at age 11 or 12.
Another viral infection associated with cancer involves the Epstein-Barr virus (EBV), a type of herpes virus. Infection with EBV can increase a person’s risk of getting nasopharyngeal cancer (which develops in the area behind the nose) and some types of lymphoma. Very few people who harbor an EBV infection will develop these cancers.
The hepatitis B and C viruses (HBV and HCV) can produce long-term infections of the liver that increase the risk of liver cancer. In the United States, less than half of liver cancers are associated with HBV or HCV infection, but the proportion is much higher in other countries where viral hepatitis and liver cancer are much more common. Both hepatitis B and C infections can be treated with drugs, lowering the risk of both liver damage and liver cancer.
The human immunodeficiency virus (HIV), which causes AIDS, doesn’t cause cancer directly, but by depleting the immune system, it can increase a person’s risk of getting a variety of types of cancer. These include Kaposi sarcoma, cervical cancer and certain kinds of non-Hodgkin lymphoma, among other cancers. While there is no preventive vaccine for HIV, taking anti-HIV drugs can help those who are infected by the virus to slow the damage to their immune system and potentially reduce their risk of developing HIV-related cancers.