Norovirus (Norwalk virus; RNA virus), animated coloured transmission electron micrograph (TEM). Norovirus is a genus of RNA viruses (family Caliciviridae), which causes about half of all gastroenteritis cases around the world. It causes approximately 90% of epidemic non-bacterial outbreaks of gastroenteritis. The Norwalk virus was originally isolated from an outbreak of the illness in Norwalk, Ohio, USA, in 1968. The disease is characterized by nausea, vomiting, diarrhoea and abdominal pain. The diarrhoea results in fluid loss and dehydration, which may become life threatening in the young, the elderly, and immunocompromised patients. The viruses are transmitted by faecal contaminated food or water and by person-to-person contact. Magnification: x21, 820 when shortest axis printed at 25 millimetres.