Norovirus (formerly Norwalk virus) is an RNA virus (taxonomic family Caliciviridae) which causes approximately 90% of epidemic non-bacterial outbreaks of gastroenteritis around the world, and may be responsible for 50% of all foodborne outbreaks of gastroenteritis in the US. Norovirus affects people of all ages. The viruses are transmitted by faecally contaminated food or water and by person-to-person contact. After infection, immunity to norovirus is usually incomplete and temporary. There is an inherited predisposition to infection, and individuals with blood type O are more often infected, while blood types B and AB can confer partial protection against symptomatic infection. Outbreaks of norovirus infection often occur in closed or semi-closed communities, such as long-term care facilities, overnight camps, hospitals, prisons, dormitories, co-ops, and cruise ships where the infection spreads very rapidly by either person-to-person transmission or through contaminated food. Many norovirus outbreaks have been traced to food that was handled by one infected person. Norovirus is rapidly inactivated by sufficient heating and by chlorine-based disinfectants, but the virus is less susceptible to alcohols and detergents as it does not have a lipid envelope.