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Helicobacter pylori, previously known as Campylobacter pylori, is a gram-negative, microaerophilic, spiral （helical） bacterium usually found in the stomach. Its helical shape （from which the genus name, helicobacter, derives） is thought to have evolved in order to penetrate the mucoid lining of the stomach and thereby establish infection. H. pylori has been associated with cancer of the mucosa-associated lymphoid tissue in the stomach, esophagus, colon, rectum, or tissues around the eye （termed extranodal marginal zone B-cell lymphoma of the cited organ）, and of lymphoid tissue in the stomach （termed diffuse large B-cell lymphoma）. H. pylori infection usually has no symptoms but sometimes causes gastritis （stomach inflammation） or ulcers of the stomach or first part of the small intestine. The infection is also associated with the development of certain cancers.