Norovirus: A group of viruses that are a common cause of food poisoning and acute gastroenteritis ("stomach flu") that can strike quickly with force and make a person feel very sick but which typically resolves within 2-3 days. The characteristic symptoms are nausea, vomiting, diarrhea, and abdominal cramping. The diarrhea is not bloody. Fever, if present, is low-grade. Dehydration is the main complication, especially in infants and the elderly, and may need medical attention.

The clinical criteria for the diagnosis of norovirus infection include:

1. an incubation period of 12-36 hours;
2. an illness characterized by acute onset of nausea, vomiting, diarrhea, abdominal cramping, and, in some cases, fever and malaise;
3. an illness of 12-60 hours duration.

The virus is spread primarily from one infected person to another (by the fecal-oral route). Infected kitchen workers can contaminate a salad or sandwich as they prepare it, if they have the virus on their hands. Infected fishermen have contaminated oysters as they harvested them. Norovirus infection has become a veritable plague on cruise ships.

Norovirus was coined to refer to the Norwalk-like virus by a simpler and more memorable term. It is now the official name. Also called the calicivirus because of its characteristic "Star of David" shape with cup-shaped (chalice) indentations.