Rotavirus infections occur repeatedly in humans from birth to old age. Most are asymptomatic or are associated with mild enteric symptoms. Infection in young children can be accompanied by severe life-threatening diarrhea, most commonly after primary infection. Annual childhood morbidity rates for severe diarrhea are similar worldwide. Mortality rates are low in developed countries but approach 1,000,000 annually in young children in developing countries. Rotaviruses can be classified into Groups A-E according to antigenic groups on VP6, the major capsid antigen. Only Group A,B and C rotaviruses have been shown to infect humans, and most human rotavirus disease is caused by Group A viruses. These are further classified into G and P types based on identification of antigens on the outer capsid proteins VP7 and VP4 respectively. Most severe infections in young children are caused by serotypes G1-4, and during the last two decades, G1 infections appear to have predominated worldwide. In general the more densely populated countries show the most complex patterns of occurrence of serotypes. Clinical rotavirus disease can be accompanied by shedding of > 10(12) rotavirus particles/gm feces. The virus is highly infectious and appears to retain infectivity over many months. In temperate climates, disease is most common during the colder months, when it is likely that rapid spread within families and communities occurs. Nosocomial infections are frequent, and rotaviruses can become endemic within obstetric hospital nurseries for the newborn. Few (if any) human rotavirus infections appear to be zoonoses, even though Group A rotaviruses are widespread in the young of all mammalian species. However infection of humans with reassortant rotavirus strains derived from human-animal sources can occur. The extent to which this contributes to new epidemic strains within particular countries (or worldwide) remains to be determined.