The name astrovirus was used by Madeley and Cosgrove in 1975 to describe a small round virus (approximately 28 nm diameter) with star-like appearance on electron microscopy. It was first seen in faeces from a few children with gastroenteritis. An aetiological role in gastroenteritis has since been confirmed. The virus causes a mild illness after an incubation period of 3-4 days. Antibody studies indicate that infection is widespread and, in Britain, mainly occurs in the 2-5 year age group. Outbreaks occur in, for example, institutions and paediatric wards. The virus usually spreads by the faecal-oral route but food- or water-borne outbreaks have occurred. Strains of astrovirus have been isolated from many animals including calf, lamb, pig, cat, dog, duck and turkey. The lamb strain can cause gastroenteritis but the bovine strain did not cause diarrhoea in gnotobiotic calves. Infected turkeys have scours, and infection in ducklings causes haemorrhagic hepatitis with a mortality up to 25%. Five human serotypes have been described, all antigenically distinct from the bovine and ovine strains. The human astrovirus does not replicate in conventional tissue cultures but undergoes a non-productive cycle in human embryo kidney cells, and productive replication in the presence of trypsin. It is a positive-strand RNA virus, which is acid stable (pH3), survives at 60 degrees C for five but not 10 minutes and, like the enteroviruses, resists inactivation by alcohols. It has a density of 1.35-1.37 g/ml in caesium chloride.