Helicobacter pylori infection has a widespread distribution but the prevalence varies considerably between developed and developing countries. The infection is probably acquired in childhood. The prevalence curves of infection according to age observed in developing countries may be explained by a generation effect, taking into account that the risk of infection has decreased in recent decades. The major risk factors are overcrowding, lack of education and poor sanitation. The exact mode of transmission is not yet understood fully but is thought to be from person to person.