Noroviruses cause more diarrhea in the U.S. than any other pathogen–probably half of all gastroenteritis worldwide. A tiny dose causes infection, then huge quanties of virons are shed in vomit and feces. This virus can survive for weeks on things like grocery bags. Swabs obtained from a bag 2 weeks after an outbreak traced to grapes and snacks carried in that bag, were still viable.
PositiveTip: Frequent hand-washing goes a long way toward preventing gastroenteritis.