Helicobacter pylori was already present in the stomach of primitive humans as they left Africa and spread through the world. Today, it still chronically infects more than 50% of the human population, causing, in some cases, severe diseases such as peptic ulcers and stomach cancer. To succeed in these long-term associations, H. pylori has developed a unique set of virulence factors, which allow survival in a unique and hostile ecological niche--the human stomach.