From “Norovirus strain strikes long-term care, cruises: a novel, highly transmissible strain emerges.” from Hospital Infection Control, March 2003 v30 i3 p34(2), one finds several alarming features of norovirus, the virus formerly known as Norwalk-like virus:
While most of the data on gastrointestinal disease outbreaks is gathered assuming that the pathogen is spread by food, other information indicates that norovirus may be more easily spread from person to person, as indicated by how common it is in closed settings like cruise ships and nursing homes. This means that information gathered so far may not be terribly useful in controlling the disease.
The virus has a low infectious dose, multiple modes of transmission, and persists in the environment. Experimental research has indicated that immunity to the virus does not persist very long.
A fact sheet from the CDC indicates that the disease is contagious from the first indication of illness to anywhere from 3 days to 2 weeks from recovery.
The virus can survive freezing, temperatures of up to 60 degrees C. and levels of chlorine in water that are higher than most municipal water system levels.
And in an interesting twist, people with type O blood may be most suceptible to infection.