Helicobacter pylori is a helical shaped Gram-negative bacterium that infects various areas of the stomach and duodenum. Many cases of peptic ulcers, gastritis, duodenitis, and perhaps cancer are caused by H. pylori infection. However, many who are infected do not show any symptoms of disease. Helicobacter spp. are the only known microorganisms that can thrive in the highly acidic environment of the stomach. Its helical shape (from which the genus name is derived) is thought to have evolved to penetrate and favor its motility in the mucus gel layer.