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. 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.