Helicobacter pylori is a type of bacteria that is a major cause of stomach (gastric) and upper small intestine (duodenal) ulcers. Infection with H. pylori may also increase the risk of stomach cancer.

H. pylori bacteria can cause ulcers by growing in the lining of the stomach, producing inflammation and causing the stomach and intestinal lining to be more easily damaged by stomach acid. But most people infected with H. pylori do not develop ulcers. An additional factor (such as stress)—one that cannot always be identified—may be needed to cause an ulcer to form.

H. pylori bacteria can be eliminated by taking certain antibiotics that are specifically directed at this bacteria.