Norovirus

Wherever large groups of people congregate in tight quarters, the dreaded norovirus has the potential to rear its ugly head; hence the close association with cruise ships, where the sickness is most likely to affect travelers. Norovirus is rarely fatal, but its effects—nausea, vomiting, and diarrhea—can really rain on the parade. And since the virus spreads through contaminated water and food, there’s little you can do to avoid it. "There’s no real prevention and no treatment, and the cruise ship industry is going crazy with how to control it," says Dr. Berger.

SEE ALSO: Travel Taboos, Do's and Don'ts