An ulcer is an open sore in the lining of the stomach or intestine, much like mouth or skin ulcers. Peptic ulcers are eventually caused by acid and pepsin, a digestive stomach enzyme. These ulcers can occur in the stomach, where they are called gastric ulcers. Or they can occur in the first portion of the intestine beyond the stomach. These are called duodenal ulcers. In the end, it is acid that causes the injury to the bowel wall. However, a revolutionary and startling recent discovery is that most peptic ulcers result from a stomach infection caused by the bacteria, Helicobacter pylori.