It is now well known that several viruses are responsible for acute diarrhoea or gastroenteritis in both children and adults. These viruses are difficult to identify since most of them cannot be isolated by stool cultures on cells. The reality of proven reinfection by some of these organisms is not always clearly understood, even though the existence of several serotypes in the same group (notably rotavirus) can be blamed, and this explains why vaccines are difficult to develop.