Most gastric adenocarcinomas and lymphomas occur in persons with current or past infection with H. pylori. In developing countries the ulcer groups are smaller and the gastric cancer group may be larger. For example, in northern Brazil, gastric cancer is the most common malignancy in men. H.pylori is uncommon in young children. Low socio-economic status predicts H.pylori infection.
Immigration is responsible for isolated areas of high prevalence in some Western countries.
In developing countries most adults are infected. It is evident from careful surveys that the majority of persons in the world are infected with H.pylori. This is evidence that spread by fecal oral contact with infected persons is likely. This may be a less common source of transmission.