A person with Norwalk virus infection has a viral infection of the stomach and intestines. The infection causes inflammation of the lining to the stomach and intestine, which results in diarrhea, vomiting, and fever.

Norwalk virus infection is an intestinal illness that often occurs in outbreaks. It is contagious because the viruses are passed in the stool of infected persons. People are infected with Norwalk virus by swallowing food or water that has been contaminated with stool from an infected person. Outbreaks in the United States are most common in those who eat raw shellfish, especially oysters and clams.