There are about 2,500 reported norovirus outbreaks in the U.S. each year. Norovirus, which is sometimes called the "cruise ship virus," causes more than 90% of diarrheal disease outbreaks on cruise ships, according to the CDC. However, norovirus outbreaks on cruise ships account for only a small percentage of all reported norovirus outbreaks.

The CDC randomly inspects cruise ships to prevent the spread of stomach viruses. Two ships received failing scores but passed in later reviews.