Sickness is defined by culture: each society has its own way of labelling, explaining and treating symptoms. In the case of childhood diarrhea, each population selects some signs and considers them as symptoms, defines a limit between normal and pathologic, arranges symptoms in order to build syndromes that make up a local nosology. Examples from Thailand, China, Algeria, Nicaragua and Burkina Faso show the diversity of popular beliefs about diarrhea. These beliefs depend upon the epidemiological context, but they do not reflect it exactly: thus in Burkina Faso, AIDS has not been integrated in popular beliefs about childhood diarrhea. The examples discussed in this article show how popular beliefs evolve, especially under the influence of biomedicine. Moreover, every individual understands differently the popular nosology according to his (or her) social status and to the level of his (her) knowledge. To the unity of biomedical knowledge responds a great diversity of popular beliefs. In France, contemporary popular beliefs about childhood diarrhea have been seldom studied by social sciences. They seem to be close to biomedical knowledge and share some essential elements with it, but they also show some features--such as the belief that diarrhea is benign when simultaneous to teething--that relate them to popular beliefs which have been described in other cultures. By describing and analysing these popular beliefs, medical anthropology makes it possible to adapt medicine to local knowledges.