Infectious Agents and Cancer

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Bacteria

Helicobacter pylori

Stomach cancer is fairly rare in the United States, but it’s the fourth most common cancer worldwide. Long-term infection of the stomach with Helicobacter pylori (H pylori) may 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.

More than half of all cases of stomach cancer are thought to be linked to H pylori infection. Still, most people who have these bacteria in their stomachs never develop cancer. There is also some evidence that people with H pylori have a lower risk of other types of cancer, although it is unclear exactly what role the bacteria plays in this.

About 1 in 3 adults has evidence of infection with H pylori, and the rate of infection is higher 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. In fact, contaminated well water has been linked to H pylori infection in the United States. 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 animals.

Antibiotics and other medicines can be used to treat H pylori infections. Doctors have given antibiotics to patients who have had superficial stomach cancers removed in order to get rid of H pylori infection. This seems to have helped prevent new stomach cancers in those patients. Patients with H pylori who have had ulcers or cancer in the lower part of the stomach should be treated to get rid of the bacteria, as should people at high risk for this type of 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. These samples are then checked for this type of bacteria. It’s a common infection in younger women who are sexually active, and can persist for years unless it’s detected and treated.

Some studies have shown 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, chlamydia may affect how long cancer-promoting HPV stays in the cervix. 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.


Last Medical Review: 02/20/2013
Last Revised: 03/07/2013