The seroprevalence of Helicobacter pylori infection increases with age. Incidence rates in children are higher than in adults and are consistent with a cohort effect reflecting primary infection during childhood. Preliminary data would suggest that horizontal transmission of infection may occur both in adults and in children. Occasionally, this may result in transient multiple infection, because the observation of mosaicism has to imply horizontal transfer of genetic information between infecting strains of H. pylori. The very close association between H. pylori infection and socio-economic conditions in childhood means that the possibility of confounding factors should be considered when studying routes of transmission or assessing the extra-gastric consequences of infection. Future studies will clarify these important issues.