The manner in which Helicobacter pylori is transmitted is of fundamental importance when considering strategies for its control, yet to date no satisfactory explanation has been advanced to account for its spread from person to person. This is surprising for a disease that affects 50% of the world's population. Attempts to culture H. pylori from faecal material and saliva have met with difficulty, casting some doubt on the likelihood of a faeco-oral or oro-oral route of transfer. The infection is easily passed from person to person by gastric intubation. The hypothesis advanced in this paper is that the natural route of transmission is by gastric juice, specifically as a result of epidemic vomiting in childhood. This theory is supported by literature indicating that acute infection with H. is characterized by vomiting of achlorhydric mucus that may serve as a vehicle for transmission. The hypothesis is consistent with most of the epidemiological data that have been published on H. pylori infection, including its association with childhood overcrowding, the lack of a fixed hot water supply and disadvantaged social conditions.