From the Association of State and Territorial Directors of Health Promotion and Public Health Education web site:

http://www.astdhpphe.org/infect/norwalk.html

Norwalk Virus Infection

Norwalk virus infection is an intestinal illness that often occurs in outbreaks.
Norwalk and Norwalk-like viruses are increasingly being recognized as leading causes of foodborne disease in the United States.
The viruses are passed in the stool of infected persons. People get infected by swallowing stool-contaminated food or water. Outbreaks in the United States are often linked to raw oysters.
Infected people usually recover in 2 to 3 days without serious or long-term health effects.

How common is Norwalk virus infection?

Norwalk and Norwalk-like viruses are increasingly being recognized as leading causes of foodborne disease in the United States. However, since no routine diagnostic test is available, the true prevalence is not known. Norwalk and Norwalk-like viruses have been linked to outbreaks of intestinal illness on cruise ships and in communities, camps, schools, institutions, and families.

Many oyster-related outbreaks of intestinal illness linked to Norwalk-like viruses have been reported in Louisiana, Florida, Maryland, and other states where oyster harvesting is common. In 1993, 73 people in Louisiana and about 130 others in the United States who ate oysters from Louisiana became ill. A malfunctioning sewage system was the cause of an outbreak in 1996. An outbreak in 1997 was linked to sewage from oyster-harvesting boats.