Acute diarrhoea is produced by many infections, and labora tory investigations are essential for detection of the causal organisms. Nevertheless, despite careful search it is impossible in the majority of cases to isolate a bacterial pathogen or to demonstrate a virus to which there is an antibody response, though it is a fair assumption that bacteria or viruses are responsible and that the emergence of new techniques will enable us to identify them with greater accuracy. On the other hand, diarrhoea and vomiting may be the presenting feature in many conditions, notably infections of the urinary tract, pneu monia, meningitis, and septicaemia. In young children short epidemics in which there is a combination of acute respiratory and gastrointestinal upset are a by no means infrequent occur rence. Diarrhoea resulting from dietetic indiscretion must also be differentiated from diarrhoea of infective origin; this may sometimes be obvious from its transient nature and the absence of pyrexia and constitutional disturbance. In Britain chronic diarrhoea is seldom due to infection, and once giardiasis has been excluded investigation for ulcerative colitis, fibrocystic disease, coeliac disease, and genetic disorders of carbohydrate metabolism should be undertaken. There are certain well-defined groups of infective diarrhoeas which may be discussed separately.