Norwalk viruses. Coloured transmission electron micrograph (TEM) of a section through two groups of Norwalk viruses. The Norwalk virus was originally isolated from an outbreak of illness in Norwalk, Connecticut, USA, in 1968. A characteristic of Norwalk viruses is their ill- defined hairy" surface. Symptoms of infection include diarrhoea and outbreaks of vomiting among humans, often in summer camps or on cruise ships. The Norwalk virus is believed to be similar to the caliciviruses, which are also implicated in human gastroenteritis and have been found in the faeces of children suffering with diarrhoea. Magnification: unknown."