Helicobacter pylori (H. pylori) is a bacteria responsible for most ulcers and many cases of chronic gastritis (inflammation of the stomach).
The bacteria can weaken the protective coating of the stomach and first part of the small intestine (duodenum), allowing digestive juices to irritate the sensitive lining of these body parts.
Gastritis - Helicobacter pylori; H. pylori
As many as half of the world's population is infected with H. pylori. Those living in developing countries or crowded, unsanitary conditions are most likely to contract the bacteria, which is passed from person to person. H. pylori only grows in the intestines, and is usually contracted during childhood.
Interestingly, many people have this organism in their gastrointestinal tract but don't get an ulcer or gastritis. It seems that other factors must also be present for the damage to take place. The factors that increase your risk for an ulcer from H. pylori include:
Ables AZ, Simon I, Melton ER. Update on Helicobacter pylori treatment. Am Fam Physician. 2007 Feb 1;75(3):351-8.