Helicobacter pylori causes chronic active gastritis and increases the risk of duodenal and gastric ulcers; persistent infection with H pylori increases the risk of gastric cancer. Acute infection can manifest as epigastric pain, nausea, vomiting, hematemesis, and guaiac-positive stools. Symptoms usually resolve within a few days despite persistence of the organism for years or for life. H pylori infection is not associated with autoimmune or chemical gastritis.