Helicobacter pylori is a common type of bacteria that usually infects people during their childhoods. In most cases, the infection does not cause problems. However, some people with H. pylori infection get inflammation or ulcers in the stomach or upper intestine. Doctors treat inflammation and ulcers caused by H. pylori with several antibiotics (for example, amoxicillin, clarithromycin, or metronidazole) for several days. Antibiotics get rid of H. pylori in most patients, but 10% to 20% of patients may still have infection despite treatment. Several factors may predispose patients to “antibiotic-resistant” infections. One possible factor is previous use of antibiotics that left behind resistant bacteria.