Helicobacter pylori (H. pylori) is a bacterium and is a major cause of peptic ulcers. It damages the mucous coating that protects the stomach and duodenum. This damage allows powerful stomach acid to irritate the lining of the stomach, and together with the H. pylori, this can cause the ulcer.
While H. pylori causes more than half of all peptic ulcers, some people infected never develop ulcers, and may never have any symptoms. In fact, many people never get sick from H.pylori and therefore do not realize they have the infection. Some people may actually be resistant to damage from H.pylori.