Reported Norovirus Outbreaks by Primary Transmission Mode and Month of Onset

The total number of norovirus outbreaks by primary transmission mode from 2009 through 2012. The number of foodborne norovirus outbreaks were low in the summer months, the greatest number occurring in January. During the same time, the number of nonfoodborne always exceeded foodborne outbreaks and was low in the summer months and dramatically higher in the winter months.