Helicobacter pylori is one of the most common bacterial infections in the world. H pylori infection of the gastric mucosa is the most common cause of peptic ulcers and is believed to be responsible for 50% to 60% of all gastric carcinomas. This infection is difficult to treat because the bacterium is located within the gastric lumen in the mucus and not within the gastric tissue. Antimicrobial therapy for H pylori includes two or three antibiotics plus either a proton pump inhibitor or a histamine receptor antagonist. H pylori readily develops resistance to antibiotics; therefore, if the initial treatment is unsuccessful, repeat treatment should include different antibiotics.