The Norovirus or so-called "cruise ship Disease" is the second only to the flu in number of cases of such diseases. Millions of Americans get it every year yet often, they don't know what they have and in most cases, within a few days it's gone so they don't bother to go to their doctor.
The ONLY reason why it's associated most often with cruise liners is, simply because the national CDC that studies such diseases used primarily cruise ships to study the problem simply because there are literally thousands of people closely associated over day or weeks at a time.
In other words, they use the Cruise industry as a sort of petrie dish.
NCL Epic 2012, Eastern Caribbean
Explorer of the Seas 2009, Eastern Caribbean
Explorer of the Seas 2007, Eastern Caribbean
Explorer of the Seas 2006, Eastern Caribbean