Most bacteria cannot survive in the stomach. Hence , H. pylori is one of the most pervasive of human pathogens because it inhabits a niche without competition. It is estimated that well over half of the world's population is infected with H. pylori. The infection rate in the United States is about 1% per year of age - for example, 30% of all 30 year-olds are infected. In developing countries , nearly all people over age 25 are infected. Only about 15%-20% of infected people to H. Pylori. What triggers the bacterial infection to become symptomatic is a major unanswered question. It seems likely that H. pylori infection and conditions that elevate acid secretion or damage the digestive tract wall contribute to the development of an ulcer. The best therapy for ulcers involving H. pylori is a combination of antibiotics and drugs that reduce the acid content of the stomach.