Infection with H. pylori is common. About two-thirds of the world’s population has it in their bodies. For most people, it doesn’t cause ulcers or any other symptoms. The most common cause of gastritis and duodenitis is a bacterium called Helicobacter pylori. When large amounts of bacteria invade your stomach or small intestine, inflammation can occur. Helicobacter pylori can be transferred from person to person. Helicobacter pylori (H. pylori) is a bacterium that causes chronic inflammation of the inner lining of the stomach (gastritis) in humans.