There's a lot of it about

From the Centers for Disease Control, Norovirus Q&A, aka the "stomach flu":

The symptoms of norovirus illness usually include nausea, vomiting, diarrhea, and some stomach cramping. Sometimes people additionally have a low-grade fever, chills, headache, muscle aches, and a general sense of tiredness. The illness often begins suddenly, and the infected person may feel very sick. In most people the illness is self-limiting with symptoms lasting for about 1 or 2 days. In general, children experience more vomiting than adults. Most people with norovirus illness have both of these symptoms.

Some salient points from the CDC information:

People infected with norovirus are contagious from the moment they begin feeling ill to at least 3 days after recovery. Some people may be contagious for as long as 2 weeks after recovery. Therefore, it is particularly important for people to use good handwashing and other hygienic practices after they have recently recovered from norovirus illness.

Anyone can become infected with these viruses. There are many different strains of norovirus, which makes it difficult for a person's body to develop long-lasting immunity. Therefore, norovirus illness can recur throughout a person's lifetime. In addition, because of differences in genetic factors, some people are more likely to become infected and develop more severe illness than others.

So a child gets sick from one strain of norovirus, then seems to get over it in about 24 hours. He goes back to school, but he's still contagious. He's also still susceptible to infection by a different strain of norovirus.

The CDC page has information on how to prevent infection and on the importance of keeping the patient hydrated. The keys to preventing infection: Wash hands frequently and clean surfaces with chlorine (bleach) based cleansers.

We've been dealing with it with one child or another for about 2 weeks, and the doctor's office says it's pretty widespread, with children going back to school before they're fully well and non-contagious.