The Norwalk virus (first identified in 1968 during an outbreak in Norwalk, Ohio) is one of several similar viruses that cause gastroenteritis (stomach flu) in both children and adults. These viruses, now known collectively as noroviruses or as caliciviruses after their biologic genus Caliciviridae, appear as small, round particles under the electron microscope (thus another alternative term: "small round-structured viruses" or "SRSVs". There are at least 4 "genogroups" (more closely genetically related) of noroviruses.
A person infected with a norovirus usually has symptoms starting 1-2 days after being exposed (most commonly the symptoms start in about 36 hours). The symptoms include nausea, vomiting, watery diarrhea without blood in the stool, and abdominal cramps. Vomiting is seen more often in affected children. A low-grade fever may also be seen. However, as many as 30% of norovirus infections may have no symptoms at all (which may make it easier for the virus to spread to others).
Like many other causes of gastroenteritis, noroviruses are passed from person to person by eating or drinking something contaminated with an infected person's stools or by direct contact with an infected person. Many outbreaks have been traced to food prepared by someone infected with the virus, and cold foods (sandwiches and salads, for example) and liquid foods such as icing or dressings are often found to be the route of transmission. (Food-related transmission probably explains the outbreaks of norovirus gastroenteritis among passengers on cruise ships where all the passengers and crew eat food prepared in the same kitchens.) Noroviruses can also survive on objects such as towels and faucet handles long enough for someone to pick them up as well. It's also possible (although not yet completely proven) that tiny droplets of the liquid vomited by an infected person can contaminate objects or even end up in a not-yet-infected person's mouth. So far, there is no evidence that you can catch norovirus by breathing droplets. It seems, though, that you only need ten virions (virus particles) to be infected. An infected person may shed norovirus particles before developing symptoms, and may shed for up to 2 weeks after getting over the symptoms.
Noroviruses are also rather robust little bugs. They can survive freezing and heat (up to 140 degrees Fahrenheit/60 degrees Celsius) -- they have actually survived steaming of shellfish long enough to make the people who ate the shellfish sick. They can also survive in water containing more chlorine than public water departments usually put in water.
The biggest problem with a norovirus infection is dehydration from water loss in the diarrheic stool. Small children and the elderly are most susceptible to dehydration. Severely dehydrated patients, especially those who can't keep fluids down because of vomiting, may need IV fluids until the vomiting and diarrhea subside. Keeping a patient hydrated is the only treatment available for norovirus infections; as with most other viruses, there are no antibiotics that work on noroviruses.
There are no vaccines available for protection against the noroviruses. It is possible that there will be no norovirus vaccines for some time; so far, being infected with one norovirus strain makes you immune only to that strain, and only for a few months. Since many strains are known to exist, this means that you can be infected many times. As with any of the food-borne infections, the best way to prevent norovirus infection is good hygeine, both in the bathroom and in the kitchen, and keeping anyone with gastroenteritis (or who have had contact with someone with gastroenteritis) away from the kitchen.