The prevalence of Helicobacter pylori infection in a community is related to three factors: firstly, the rate of acquisition of infection with H. pylori--that is, incidence; secondly, the rate of loss of the infection; thirdly, the prolonged persistence of the bacterium in the gastroduodenal mucosa between infection and eradication. Variation in the prevalence of H. pylori is dominated by the great differences between communities in the incidence of H. pylori infection during childhood. The countries of the world form two groups: Group One is made up of those where the majority of children become infected with H. pylori during childhood and chronic infection continues during adult life; in Group Two only a minority of children are infected during childhood, but the prevalence of infection rises in proportion to age during adult life. Understanding the ages at which people acquire infection with H. pylori is crucial to the interpretation of H. pylori prevalence data.