In the developing countries diarrhoea ranks among the most frequent diseases: 5-18 million children are estimated to die annually from gastro-intestinal infections. But also in Europe and the USA diarrhoea is of utmost medical importance, especially among children and infants. It was only twelve years ago that 2 viral groups, the Norwalk and the rotavirus group, were discovered to be etiological agents responsible for a large proportion of gastro-intestinal infections. Whilst viruses of the Norwalk group cause primarily gastro-intestinal infections in schools and families (school-children and adults), rotaviruses attack mainly infants. In our latitudes rotaviruses are important agents responsible for nosocomial infections. A rapid diagnosis is crucial if the chain of infections is to be interrupted. Uncontrolled spread may not only lead to severe disease in infants and young children, but also burdens health insurance - plans considerably. Experiments intended to develop a rotavirus vaccine are under way.