The human infant in the first six months of life is particularly vulnerable to diarrhoeal disease. Vulnerability to diarrhoeal processes is associated with the state of development of the infant gastrointestinal tract, not only of its digestive and absorptive processes but also of its unique and elaborate local defence system, consisting of both immune and non-immune elements. Protection from enteric infection, and sensitisation to food antigens is a function of the integration of gastric acid and biliary secretion, intestinal motor activity and local immune mechanisms. Development of these functions is controlled by a species specific programme in which the potential for accelerated development appears limited. Human milk provides passive protection during the initial vulnerable period. However, premature weaning may at best result in overloading of digestive and absorptive mechanisms and has the potential for the development of sensitisation to food antigens and serious diarrhoeal disease.