The virus is very small and often difficult to detect. Norwalk viruses are more commonly associated with outbreaks in nursing homes, homes for the aged and other adult institutions due to the close proximity of the residents. Outbreaks have also been linked to banquets, cruise ships, swimming pools, schools and restaurants. Man is the only known reservoir for the Norwalk virus. The virus is very contagious and only a few are needed to cause illness.
Contaminated water is the most common source for outbreaks other than in institutions.
Shellfish and salad ingredients are the foods most often implicated in Norwalk outbreaks. Immunity against the virus is usually short-term, about 14 days. The incidence of illness can occur year round however, it is higher in the fall and winter. Thorough hand washing is the best prevention. Make sure hands are properly washed after using the toilet, changing diapers and preparing food.