H. pylori, first described in 1984, is a gram-negative spiral-shaped bacillus that lives in the mucus layer on the surface of the gastric mucosa where it causes gastritis. In most people the chronic gastritis is asymptomatic. In some cases, altered gastric secretion and tissue injury leads to peptic ulcer disease, or gastritis progresses to atrophy, intestinal metaplasia and eventually gastric carcinoma or rarely, gastric lymphoma. It is estimated that H. pylori-positive patients have a 10-20% lifetime risk of developing peptic ulcer disease and a 1-2% risk of developing distal gastric cancer.
The organism, confined almost solely to humans and primates, is commonly acquired in childhood and persists for life unless specifically treated. In developing countries, more than 80% of the population may be infected. In industrialized countries, the prevalence of H. pylori is low in childhood and rises with increasing age so that 20-50% of adults are likely to be infected. The increased prevalence of infection with age was initially thought to represent a continuing rate of acquisition throughout one's lifetime, but is now thought to reflect that particular cohort's rate of bacterial acquisition during childhood years.
The route by which infection occurs remains unknown. Person-to-person transmission of H. pylori through either fecal-oral or oral-oral exposure seems most likely.