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Cancer Risk and Prevention

Bacteria that Can Lead to Cancer

Bacteria are very small living things that are made up of only one cell. Most types of bacteria aren’t harmful, but some can infect people and cause diseases. A few have even been linked with cancer.

Helicobacter pylori

Stomach cancer is not common in the United States, but it’s one of the more common types of cancer worldwide. Long-term infection of the stomach with Helicobacter pylori (H pylori) can cause ulcers. It can also inflame and damage the inner layer of the stomach. Some of these changes could lead to cancer over time, especially cancer in the lower part of the stomach. H pylori infection is also linked with some types of lymphoma of the stomach.

While H pylori infection is a major cause of stomach cancer, most people who have these bacteria in their stomachs never develop stomach cancer. There is also some evidence that people with H pylori might have a lower risk of some other types of cancer, although it is unclear exactly what role the bacteria plays in this.

About 2 in 3 adults worldwide are infected with H pylori. The rate of infection is higher developing countries and in older age groups. It’s likely spread in a couple of ways. One is the fecal-oral route, such as through contaminated food or water sources. It can also be transmitted from one person to another, mouth to mouth.

Other factors also play a role in whether or not someone develops stomach cancer. For example, nitrites are substances commonly found in cured meats, some drinking water, and certain vegetables. They can be converted by certain bacteria, such as H pylori, into compounds that have been found to cause stomach cancer in lab animals.

Antibiotics and other medicines can be used to treat H pylori infections. According to the CDC, people who have active ulcers or a history of ulcers should be tested for H pylori, and, if they are infected, should be treated. Testing for and treating H pylori infection is also recommended after removal of an early stomach cancer.

Chlamydia trachomatis

Chlamydia trachomatis is a very common kind of bacteria that can infect the female reproductive system as well as other parts of the body in both men and women. It is spread through sex.

Although infection of the reproductive organs may cause symptoms in some people, most women have no symptoms. This means that women with chlamydia usually don’t know they’re infected unless samples are taken during a pelvic exam and tested for chlamydia. It’s a common infection in younger women who are sexually active, and can remain for years unless it’s detected and treated.

Some studies have found that women whose blood tests showed past or current chlamydia infection may be at greater risk for cervical cancer than women with negative blood test results.

Studies have not shown that chlamydia itself causes cancer, but it might work with HPV in a way that promotes cancer growth. For example, researchers have found that women who had chlamydia along with HPV are more likely to still have HPV when they are re-tested later than women who have not had chlamydia. Although more studies are needed to confirm these findings, there are already good reasons to be checked for chlamydia infection and have it treated with antibiotics if it is found.

In women, long-term chlamydia infection is known to cause pelvic inflammation that can lead to infertility, mainly by building up scar tissue in the Fallopian tubes. Like other infections that inflame or cause ulcers in the genital area, chlamydia can also increase the risk of becoming infected with HIV during exposure to an HIV-infected sexual partner.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.

Anttila T, Saikku P, Koskela P, et al. Serotypes of Chlamydia trachomatis and risk for development of cervical squamous cell carcinoma. JAMA. 2001;285:47–51.

Brown LM. Helicobacter pylori: epidemiology and routes of transmission. Epidemiol Rev. 2000;22:283–297.

National Cancer Institute. Helicobacter pylori and Cancer. 2013. Accessed at on September 22, 2014.

Silins I, Ryd W, Strand A, et al. Chlamydia trachomatis infection and persistence of human papillomavirus. Int J Cancer. 2005;116:110–115.

Last Revised: July 11, 2016

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