<Tr> <Th> Frequency </Th> <Td>> 50% </Td> </Tr> <Tr> <Td_colspan="2"> (edit on Wikidata) </Td> </Tr> <P> Helicobacter pylori, previously Campylobacter pylori, is a gram - negative, microaerophilic bacterium found usually in the stomach . It was identified in 1982 by Australian scientists Barry Marshall and Robin Warren, who found that it was present in a person with chronic gastritis and gastric ulcers, conditions not previously believed to have a microbial cause . It is also linked to the development of duodenal ulcers and stomach cancer . However, over 80% of individuals infected with the bacterium are asymptomatic, and it may play an important role in the natural stomach ecology . </P> <P> More than 50% of the world's population harbor H. pylori in their upper gastrointestinal tract . Infection is more common in developing countries than Western countries . H. pylori's helical shape (from which the genus name derives) is thought to have evolved to penetrate the mucoid lining of the stomach . </P>

Where does the bacteria that causes ulcers come from