Last year, there were 16 outbreaks on cruise ships, from norovirus, salmonella, E. coli or other unidentified pathogens that cause gastrointestinal symptoms such as diarrhea or vomiting. That's the largest number of cruise ship outbreaks in 12 years.

What's sneaky, super contagious and bound to cause diarrhea and vomiting? If you answered "norovirus," you'd be correct. But what is it, and why is it so often associated with cruises?

In December alone five cruise ships from Cunard, Holland America Line and Princess Cruises, all Carnival-owned brands, reported norovirus outbreaks affecting hundreds of passengers. An outbreak is declared when 3% or more of a ship's passengers and crew come down with gastrointestinal problems.

Norovirus, often coined the “cruise ship virus,” causes more than 90% of diarrheal disease outbreaks on cruise ships, according to the CDC. Still, norovirus outbreaks on cruise ships account for only a small percentage of all reported norovirus outbreaks.