"tis the season........

I am hearing about schools around the country with noro outbreaks. I remember when my childrens school had one a few years ago. How does it happen so fast. It is like one day there will be a few kids out. Then the next day 100 and the next day 150!!! I know the illness is allegedly spread by the fecal-oral route. If that is the case, how can so many be sick and so quickly?

Stella