Helicobacter pylori: A bacterium that causes stomach inflammation (gastritis) and ulcers in the stomach and duodenum. This bacterium is the most common cause of ulcers worldwide. H. pylori infection may be acquired from contaminated food and water or through person-to-person contact. It is common in people who live in crowded conditions with poor sanitation. This bacterium is also believed to be associated with stomach cancer (gastric adenocarcinoma) and a rare type of lymphoid tumor called gastric MALT lymphoma. Infected persons usually carry H. pylori indefinitely, often without symptoms, unless treated with antibiotics to eradicate the bacterium. Also known as ulcer bug.