Bacteria

Helicobacter pylori

Stomach cancer is fairly rare in the United States, but it is 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.

About 1 in 3 adults has evidence of infection with H pylori, and the rate of infection is higher in older age groups. Researchers aren’t exactly sure how H pylori may be spread from one person to another, but a likely route of spread is through a fecal-oral route, such as through contaminated water sources. In fact, contaminated well water has been linked to H pylori infection in the United States. Because the bacteria’s DNA is found in saliva, it can also be transmitted from 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 prevented 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. It is spread through sex. Although infection of the reproductive organs may cause symptoms in some people, more than 2 out of 3 women have no symptoms. This means that most women with chlamydia do not know they are infected unless samples are taken during a pelvic exam. These samples are then checked for this type of bacteria. It is very common in younger women who are sexually active, and may persist for years unless it is detected and treated.

Some studies have shown that women whose blood was found to show past or current chlamydia infection may be at greater risk for cervical cancer than women with negative blood tests. Studies have not shown that chlamydia by itself can cause cancer. But it may work with HPV in some way that promotes cancer growth.

One possible explanation suggested by more recent studies is that chlamydia may affect how long cancer-promoting HPV stays in the cervix. Researchers found that women who had chlamydia along with HPV were more likely to still have HPV when they were re-tested later than the women who had 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.

Long-term chlamydia infection is known to cause of pelvic inflammation that can lead to infertility, mainly by building up scar tissue in the Fallopian tubes. Like other sexually transmitted diseases that irritate or ulcerate the genital area, chlamydia can also increase the risk of becoming infected with HIV during exposure to an HIV-infected sexual partner.