<qa>  What is the most important cause of non-bacterial epidemics?Noroviruses have been recognized as the most important cause of non-bacterial epidemics of acute gastroenteritis, affecting individuals of all ages, in both developing and developed countries. Such outbreaks usually have high attack rates and have occurred in child care centers, schools, restaurants, summer camps, hospitals, nursing homes, ships both civilian and military as well as deployed military troops. The viruses are highly contagious and are able to survive in the environment for extended periods of time. They are spread by contact with environmental surfaces and person-to-person transmission. Noroviruses normally cause moderate-to-severe diarrhea and the disease is often incapacitating and can be fatal in the young and elderly. The most important public health concern is that Noroviruses can cause large water- and food-borne outbreaks, resulting in the virus being classified as a Category B agent. 