The unique reservoir of Helicobacter pylori is the stomach of humans. Several animals which were considered as potential reservoirs (e.g. pig, cat, sheep, monkey) have been exonerated. Flies and cockroaches could be implicated if the bacterium is eliminated in huge numbers and in viable forms in feces. The potential sources of contamination are vomit, saliva and feces. In infected subjects, H. pylori is always present in vomit and can survive a few hours in the environment. Saliva is sometimes positive because of regurgitations, but feces only contain viable organisms when there is a short transit and even so, this finding is not constant. Transmission occurs essentially in childhood and is mostly intra-familial. In developing countries the fecal-oral route is plausible, in addition to the oral-oral route; indeed, diarrhea is common, fecal hygiene is rarely performed and water is not treated. Furthermore, when vomiting occurs, hygienic practice cannot be adequate due to the unsatisfactory sanitary conditions at home, and certain habits, e.g. premastication of food, are frequent. To the contrary, in developed countries, factors for a fecal-oral transmission seems to have disappeared, which is in agreement with a currently rare transmission rate, and probably essentially via the oral-oral route.