Viral gastroenteritis is highly contagious. The viruses are commonly transmitted by people with unwashed hands. People can get the viruses through close contact with infected individuals by sharing their food, drink, or eating utensils or by eating food or drinking beverages that are contaminated with the virus. Noroviruses, in particular, are typically spread through contact with the stool or vomit of infected people and through contaminated water or food?especially oysters from contaminated waters.
People who no longer have symptoms may still be contagious, since the virus can be found in their stool for up to two weeks after they recover from their illness. Also, people can become infected without having symptoms and can still spread the infection.
Outbreaks of viral gastroenteritis can occur in households, child care settings, schools, nursing homes, cruise ships, camps, dormitories, restaurants, and other places where people gather in groups. If you suspect that you were exposed to a virus in one of these settings or by foods prepared on the premise of places such as a restaurant, a deli, or a bakery, you may want to contact your local health department, which tracks outbreaks.
This answer is based on source information from National Institute of Diabetes and Digestive and Kidney Diseases.