Noroviruses are the leading cause of reported outbreaks of gastroenteritis (vomiting, diarrhea, and stomach cramping caused by inflammation of the stomach and intestines). While the vast majority of norovirus illnesses are not part of a recognized cluster, outbreaks provide important information on how the virus spreads and, therefore, how best to prevent infection.
Norovirus outbreaks occur throughout the year. But, over 80% of the outbreaks occur from November to April. Also, when there are new strains of norovirus, the number of outbreaks can sometimes increase