Helicobacter pylori (H. pylori) is a bacterium that lives in the stomach of many people and can sometimes cause inflammation of the stomach (gastritis), ulcers, and in some cases (in about 1 out of 100 people with the infection) stomach cancer. The vast majority of stomach cancers are caused by H. pylori. The frequency of H. pylori infection and the number of new cases of stomach cancer have declined in many areas of Europe, but there are still high-risk areas, particularly in eastern Europe and parts of Spain, Portugal, and Italy.