Infection with Helicobacter pylori, a type of bacteria, is a major cause of stomach ulcers and ulcers of the duodenum. It may also increase the risk of developing stomach cancer. In some people, H. pylori bacteria grow in the lining of the stomach and duodenum and may cause inflammation. This inflammation can increase a person's risk of developing an ulcer. However, not all people infected with H. pylori develop ulcers. H. pylori can often be transmitted to humans from pets.