Abstract Helicobacter pylori infection is one of the most common human infections in the world. The bacteria cause peptic ulcer disease and their infection is an important factor for gastric cancer development. The bacteria are transmitted from person to person within families, with young children most often infected. The bacteria reside in the stomach for a lifetime if untreated by antimicrobial agents. Many virulence factors are known that contribute to the persistence of the bacteria in the stomach, and the bacteria harbors a pathogenicity island in its genome. The discovery of H. pylori as a cause of gastritis in the stomach led to the Nobel Prize in Physiology or Medicine 2005.