Helicobacter pylori (H pylori) is a common bacterium that is present in millions of people worldwide. In the United States, more than 50% of people older than 60 years are affected. Helicobacter pylori is found in the mucous lining of the stomach. It is known to be responsible for 60% to 80% of gastric ulcers (those occurring in the stomach) and 70% to 90% of duodenal ulcers (those occurring in the first part of the small intestine). The recognition of an association between this bacterium and peptic ulcer disease (occurring in the stomach or duodenum) by Dr Barry J. Marshall and Dr John R. Warren, both from Australia, was made in 1983, and they were awarded the Nobel prize in physiology (how the body works internally) and medicine in 2005. It is now understood that peptic ulcer disease is not caused by stress or by eating foods high in acid. It is often caused by the H pylori bacterium. Although the infection typically is acquired in childhood, it may continue without any problem throughout a person's lifetime. Only 15% to 20% of those who carry this bacterium ever develop symptoms related to an ulcer. Treatment is recommended only for people who have an ulcer as a consequence of the bacterium's presence. Treatment is important because people who have ulcers tend to have recurrent problems, and eliminating the H pylori infection prevents relapses. Patients infected with H pylori are more likely than others to develop certain types of cancer of the stomach, so testing and treatment for H pylori infection is recommended in patients with a family history of gastric cancer. This issue of JAMA includes a Commentary about a new treatment option for H pylori.