Peptic ulcer disease is a common clinical ailment, once thought to be caused by over secretion of acid and pepsin, an enzyme of the stomach that promotes digestion by breaking down proteins. Researchers have found, however, that although the injury caused by acid and pepsin is necessary for the formation of ulcers, acid secretion levels of the majority of patients with gastric or duodenal ulcers are normal. An ulcer is now known to be the result of an imbalance between aggressive and defensive mechanisms in the stomach and duodenum. Part of that imbalance can be attributed to infection by H.pylori.