Norwalk virus: A family of small round viruses that are an important cause of viral gastroenteritis (viral inflammation of the stomach and intestines). Norwalk disease is a significant contributor to illness in the US. Only the common cold is reported more frequently as a cause of disease. About a third of all cases of viral gastroenteritis after infancy are due to Norwalk viruses.
The Norwalk virus family contains a number of agents named after the places where outbreaks occurred. The Norwalk, Montgomery County, Snow Mountain (California) and Hawaii agents were identified in the US; the Taunton, Moorcroft, Barnett, and Amulree agents in the UK; and the Sapporo and Otofuke agents in Japan.
The Norwalk viruses are all transmitted by fecal contamination of water and foods. Person-to-person transmission is far less common but has been documented. Water is the most common source of outbreaks and may include water from municipal supplies, wells, lakes, pools, and water stored aboard cruise ships. Shellfish and salad ingredients are the foods most often implicated. Eating raw or insufficiently steamed clams and oysters poses a high risk for infection with Norwalk virus.
The percentage of individuals in developing countries who are immune at an early age is very high (as it is with poliovirus and other viruses transmitted by fecal contamination of water and foods). In the US the percentage increases gradually with age, reaching 50% in people over 18 years of age. Immunity, however, is not permanent and reinfection can occur.
Norwalk disease is a mild and brief illness that develops 1-2 days after contaminated food or water is consumed and lasts for 24-60 hours. Symptoms include nausea, vomiting, diarrhea, abdominal pain and upon occasion headache and low fever. Severe illness requiring hospitalization is most unusual.