Helicobacter pylori and Other Bacteria That Can Lead to Cancer
Some types of bacteria have been linked to cancer in humans. Learning about which bacteria cause cancer might help you lower your risk.
What are bacteria?
Bacteria are tiny germs that are made up of only one cell. Most types of bacteria aren’t harmful, but some can infect people and cause diseases, including cancer.
Helicobacter pylori (H. pylori)
H. pylori is a type of bacteria that can cause stomach ulcers and gastritis (inflammation that damages the inner stomach lining). Over time, damage to the stomach lining caused by chronic infection can increase cancer risk, especially in the lower part of the stomach (called the non-cardia stomach).
H. pylori and cancer
Although most people who have an H. pylori infection never develop cancer, long-term infection is linked with stomach cancer and a rare type of lymphoma that starts in the stomach known as gastric MALT lymphoma.
Some research suggests that people who have an H. pylori infection might have a lower risk of esophageal cancer. This might be because H. pylori infection often reduces stomach acid levels. This might reduce acid reflux – a risk factor for esophageal cancers. Even so, this possible effect does not outweigh the well-established harms of chronic H. pylori infection.
Spread, testing, and treatment
H. pylori mainly spreads through contact with infected stool, saliva, or vomit and can also spread through contaminated food or water.
According to the Centers for Disease Control and Prevention (CDC), people should be tested for H. pylori if they have or have had:
- Stomach ulcers
- Early-stage stomach cancer removed
If an infection is found, it should be treated. Antibiotics and other medicines can be used to treat H. pylori infections, which helps lower the risk of stomach cancer.
Other bacteria linked to cancer
Researchers are studying whether Chlamydia trachomatis might contribute to cancer risk in certain situations. These links are less firmly established than the link between H. pylori and stomach cancer.
Chlamydia trachomatis
Chlamydia trachomatis (chlamydia) is a type of bacteria. It is one of the most common sexually transmitted infections (STIs).
Symptoms and treatment
Although chlamydia can cause symptoms, most people have no symptoms when infected. Because of this, the infection often goes unnoticed unless STI testing is done.
In women, chlamydia can lead to pelvic inflammatory disease (PID). This can cause inflammation and scar tissue in the fallopian tubes, which might lead to infertility or an ectopic pregnancy.
If a chlamydia infection is found, it’s best treated with antibiotics.
Chlamydia and cancer
Some studies suggest that women who have had chlamydia might be at a higher risk for cervical cancer. Although chlamydia does not directly cause cancer, it might promote cancer growth in women who are also infected with human papillomavirus (HPV). Some studies have also suggested that past chlamydia infection might be linked with a higher risk of ovarian and other gynecologic cancers.
In men, untreated chlamydia infection can lead to inflammation in or around the prostate gland, which in theory might increase the risk for prostate cancer.
However, more studies are needed to confirm the possible links between chlamydia and cancer risk.
- Written by
- References
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.
Baden LR, Swaminathan S, Almyroudis NG, et al. Prevention and Treatment of Cancer-Related Infections, Version 3.2024. NCCN Clinical Practice Guidelines in Oncology. J Natl Compr Canc Netw. 2024;22(9):617-644.
National Cancer Institute. Helicobacter pylori and Cancer. 2023. Accessed at www.cancer.gov/cancertopics/factsheet/risk/h-pylori-cancer on February 16, 2026.
Pillai AB, Wong CM, Abidin ND, et al. Chlamydia infection as a risk factor for cervical cancer: A systematic review and meta-analysis. Iranian journal of public health. 2022 Mar;51(3):508.
Ramadan YN, Alatawi MN, Albalawi AS, et al. Bacterial contributions to cancer development: mechanisms, dysbiosis, and cross-cancer associations. Infect Agents Cancer 21, 11 (2026).
Last Revised: August 12, 2026
American Cancer Society medical information is copyrighted material. For reprint requests, please see our Content Usage Policy.
American Cancer Society Emails
Sign up to stay up-to-date with news, valuable information, and ways to get involved with the American Cancer Society.
