The nasty bug that sickened vacationers on several cruise ships recently is one of several strains of the Norwalk virus. Infection with it causes viral gastroenteritis, marked by nausea, vomiting, diarrhea, and abdominal cramps that last for two or three days.
Norwalk viruses are highly contagious so it isn't surprising that so many people were infected at the same time on the cruise ships (although those affected were only a small percentage of the several thousand passengers the ships carry). Outbreaks typically occur in places that people gather in close proximity to each other for a few days in a row - schools, hotels, camps, nursing homes, hospitals. The cruise ship outbreaks have received a lot of publicity, but every year an estimated 181,000 people in the United States develop these infections.
The means of transmission is the fecal/oral route - infected people (whether or not they have symptoms) shed the virus in their stool and may contaminate their hands when wiping themselves. If they don't wash their hands properly, they can spread the virus to everything and everyone they touch. That includes doorknobs, elevator buttons, even food. The virus can survive on any of these objects for as long as 12 days, plenty of time for others to pick it up by touching an elevator button or turning a doorknob and then using their hands to pick up food. Someone who has had the virus can continue to shed it two weeks after recovery. And, unfortunately, there are so many strains of it that catching one won't leave you immune to any of the others.
Your best defense against the Norwalk virus is to wash your hands often and thoroughly with warm water and soap and to be very careful not to put your hands in your mouth. You're unlikely to pick up the virus if you can avoid places where it is passed around. But that's not a practical solution for most of us. Unpleasant though they are, these infections are fortunately short-lived.
Andrew Weil, M.D.