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.

Beth Knowles and her family left on a Caribbean cruise in late December, but they missed every port. They got too sick to disembark during a norovirus outbreak on board. Knowles got an email from Cunard Line one day before their Dec.

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?