The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause 23 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in the United States (CDC, 2006; Fankhauser, et al., 2002; Mead, et al., 1999). Of viruses, only the common cold is reported more often than viral gastroenteritis (norovirus) (Benson & Merano, 1998).
Noroviruses are transmitted primarily through the fecal-oral route, and fewer than 100 norovirus particles are said to be needed to cause infection (MMWR, 2001, June 1).

Transmission occurs either person-to-person or through contamination of food or water. Foodborne norovirus transmission can occur when food is contaminated by an infected food handler (Caceres, et al., 1998; MMWR, 2001, June 1).

Noroviruses are recognized as causing over half of all foodborne illness outbreaks. CDC statistics show that food is the most common vehicle of transmission for noroviruses; of 232 outbreaks of norovirus between July 1997 and June 2000, 57% were foodborne, 16% were spread from person-to-person, and 3% were waterborne (CDC, 2006, August 3).

The virus is shed in large numbers in the vomit and stool of infected individuals, most commonly while they are ill. Some individuals may, however, continue to shed norovirus long after they have recovered from the illness (Patterson, 1993). Aerosolized vomit has also been implicated as a mode of norovirus transmission (Marks, et al., 2000).
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Shelby Bailey, RS

Health Environmentalist