What Dave is saying is that while the report sounds bad -100 people reported to have the norovirus - That's a relatively small outbreak considering the close confines of a cruise ship. If you realize there are 3000 or so people on board (more if you include crew) its not a big percentage. Also realize that the ship reports and treats any guest that goes to the infirmary with stomach problems as a norovirus case whether it truly is or not.
I would be most people would be shocked at the number of people that come down with similar symptoms say at Disney World or a land resort. But those numbers are not known since its only the cruise ships that are forced to track and report the problem.
The cruise lines take an extreme stance of prevention against the virus. Mainly because having a big outbreak causes alarm. When there is an outbreak, extra cleaning and disinfection procedures are taken between cruises. I would not worry much about what the previous cruise had unless it was a much larger percentage of the passengers.
Take precautions - wash your hands more than you would at home, (especially before eating) and use the sanitizer dispensers provided and you should have no problem. In fact if everyone followed those precautions, there would be a lot less cases reported.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007