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?

Norovirus causes an inflammation of the stomach or intestines called acute gastroenteritis, according to the CDC. The virus is “very contagious,” and symptoms include vomiting, diarrhea, stomach pain, fever, headache and others.

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.