Infantile acute gastroenteritis is still a frequent problem particularly in younger children, with high mortality rate in developing countries and high impact on health costs in industrialized countries. The increased knowledge on its pathophysiology has led to the definition of two distinct mechanisms of diarrhea: the secretory and the osmotic pathway. Investigation on the host-microorganism interaction revealed a complex scenario with sophisticated mechanisms developed by microorganisms during evolution to overcome the host defense system. The latter includes immune and non immune coordinated components, with a major role played by the GALT (gut associated lymphoreticular tissue). Knowledge of epidemiology and of the natural history of intestinal infections has led to rational diagnostic approach with substantial cut of medical costs. Novel therapeutic strategies have been made available with the use of probiotics and of passive immunotherapy together with a dramatic reduction of antibiotic treatment. HIV pandemy raises major problems which need rapid responses.