Helicobacter pylori is a bacterium (single-celled organism) that lives deep in the mucous layer lining of the stomach of some people. Infection with H. pylori leads to inflammation of the lining of the stomach (gastritis) and in some infected people, upper intestinal (duodenal) or stomach (gastric) ulcers. The infection may also play a role in the development of stomach tumours. It is more common in older people and is much less frequent in those born more recently. It persists unless it is treated with medications.

the type species of genus Heliobacter; produces urease and is associated with several gastroduodenal diseases (including gastritis and gastric ulcers and duodenal ulcers and other peptic ulcers)

A bacteria normally found in the digestive system; when present in large enough numbers, the bacteria causes most cases of ulcers in the stomach and duodenum.