H. Pylori causes stomach ulcers. Most people probably have some H. Pylori in their stomach, which is not a problem, unless your immune system is fucked, then it can get out of control and cause stomach ulcers. Some Australian guys won the Nobel prize in medicine for showing that stomach ulcers can be treated with anti-biotics and were not caused by stress. (Although, the reality is that stress can effect your immune system, which is likely a common way that H.Pylori gets out of hand, so the idea that stomach ulcers are caused by stress is not entirely wrong).