virus caused by Norwalk virus, a 27-nm RNA virus in the family Caliciviridae frequently occurring in a group of people (e.g., in a school or small community) suddenly and without prodromal illness or malaise, is intense while it lasts, but ceases abruptly after 24-48 hours; symptoms are headache, abdominal pain, giddiness, and diarrhea in most of the cases, and extreme prostration in about 75%. SYN: epidemic nausea.
