Helicobacter pylori is a pathogenic bacteria that always causes an inflammatory reaction in the gastric mucosa. Gastric inflammation or gastritis associated with H. pylori is asymptomatic in most cases. H. pylori is the cause of more than 70% of gastroduodenal ulcers, and they disappear on its eradication. MALT (mucosa-associated lymphoid tissue)-type small-cell gastric lymphoma may regress completely after H. pylori eradication. H. pylori gastritis may lead to intestinal atrophy and metaplasia and to gastric cancer, in 1% of cases. H. pylori eradication makes it possible to stabilize the precancerous lesions and prevent the onset of cancer. The risk of cancer depends on the bacteria's pathogenicity and the host's immune characteristics. In France, preventive eradication is recommended in first-degree relatives of subjects with gastric cancer, patients with a partial gastrectomy for cancer and patient with marked atrophy.