Helicobacter pylori (H. pylori) is a type of bacteria. Researchers believe that H. pylori is responsible for the majority of peptic ulcers. H. pylori infection is common in the United States: About 20 percent of people under 40 years old and half of those over 60 years have it. Most infected people, however, do not develop ulcers. Why H. pylori does not cause ulcers in every infected person is not known. Most likely, infection depends on characteristics of the infected person, the type of H. pylori, and other factors yet to be discovered.

Researchers are not certain how people contract H. pylori, but they think it may be through food or water. Researchers have found H. pylori in the saliva of some infected people, so the bacteria may also spread through mouth-to-mouth contact such as kissing.

Helicobacter pylori has a unique way of adapting in the harsh environment of the stomach. It protects itself by covering itself with the mucus of the stomach. Once it is covered with mucus, it is able to fight the stomach acid that does reach it with an enzyme it possesses called urease. Because of its spiral shape and the way it moves, H. pylori can penetrate the protective lining of the stomach. When it penetrates the stomach, excess acid can irritate the stomach and duodenum (pronounced Doo-o-DEE-num) eventually causing an ulcer.

When an ulcer has healed, it is likely to come back within a year. When H. pylori is completely eliminated from the digestive tract, the chances that the ulcer will return are greatly reduced.