Norovirus outbreaks increase during the winter months because more people are together indoors. But some venues, such as nursing homes, schools, and cruise ships, can be struck by this virus at any time. A Norovirus outbreak on a Royal Caribbean cruise ship has sickened 475 passengers.
According to the Centers for Disease Control and Prevention (CDC), 129,678 passengers were sickened on cruise ships from 2008 to 2014. That seems like a large number, but just 10% of those illnesses were norovirus. And the number of people on those cruses was 74,000,000