The Washoe District Health Department reported that many of the 24 people that became sick on the Red Rock and Trailer 1 fires northwest of Reno, Nevada last week tested positive for the norovirus. The exact numbers were not available, but not everyone with symptoms gave a sample.
Here is a portion of the Wikipedia entry for norovirus:
Norovirus (formerly and still sometimes referred to as the Norwalk virus) is an RNA virus of the Caliciviridae taxonomic family. This virus causes approximately 90% of epidemic non-bacterial outbreaks of gastroenteritis around the world, and may be responsible for 50% of all foodborne outbreaks of gastroenteritis in the US. Norovirus affects people of all ages. The viruses are transmitted by faecally contaminated food or water and by person-to-person contact.
After infection, immunity to norovirus is usually incomplete and temporary. There is an inherited predisposition to infection, and individuals with blood type O are more often infected, while blood types B and AB can confer partial protection against symptomatic infection.
Outbreaks of norovirus infection often occur in closed or semi-closed communities, such as long-term care facilities, overnight camps, hospitals, prisons, dormitories, and cruise ships where once the virus has been introduced, the infection spreads very rapidly by either person-to-person transmission or through contaminated food. Many norovirus outbreaks have been traced to food that was handled by one infected person.
Norovirus is rapidly inactivated by chlorine-based disinfectants, but because the virus particle does not have a lipid envelope, it is less susceptible to alcohols and detergents.