this is spreading faster than we can stop it. >> the crews are making calls and cutting the trip short. they are two days early and expected to dock on wednesday. the medical correspondent, you feel for the people who took the time off from work and now they are sick with a mystery illness. still a mystery? >> i suppose you can call it a mystery, but doctors say it's norovirus that happens with some regularity on cruise ships. the gastro intestinal illness. i won't go through the details. >> that's one characteristic of norovirus. >> so on a cruise, i have only been on one, but tight quarters. how do you prevent it or at least slow the spread. can you? >> you can wash your hands a lot. there a lot of hand sanitizer stations where you can clean up. that's one of the few ways.