norovirus or Norwalk virus, highly contagious viral disease caused by infection with an RNA virus of the genus Norovirus. The virus causes acute gastroenteritis, usually one to two days after infection; typical symptoms are abdominal pain and cramps, diarrhea, vomiting, and a low-grade fever. Dehydration may result if enough liquids are not consumed to replace lost fluids. There is no treatment for norovirus, which usually lasts one to three days and does not result in severe illness, but it may become life-threatening if an infected person becomes dehydrated. Dehydration may be treated by administering fluids intravenously. A person typically remains contagious for several days after the acute illness has ended.

Norovirus is transmitted by the fecal-oral route, most commonly through person-to-person contact a result of poor hygiene or through food contaminated by an infected food preparer or handler. Ingestion of uncooked shellfish harvested from contaminated waters or of contaminated drinking water may also transmit the disease. Outbreaks most commonly occur in settings where individuals are in close quarters, such as hospitals, nursing homes, and retirement homes, vacation resorts and cruise ships, and child-care centers and schools, and spread rapidly; most epidemic viral gastroenteritis is caused by a Norovirus. The virus can persist in the environment on some surfaces for a relatively extended period of time, and is resistant to disinfection by alcohol- or detergent-based agents, but soap and water is effective in blocking the spread of the disease. The disease was first identified as result of an outbreak in a Norwalk, Ohio, elementary school in 1968.