Depends on the cruise line, and sometimes even the specific ship. Disney Cruise Line keeps everything spotless (they're one of the only cruise lines with a perfect CDC inspection record year after year) and makes everyone wash their hands with actual soap and water before entering the buffet line. Norovirus outbreaks are exceedingly rare on DCL, though airborne vectors for stuff like the flu are pretty much impossible to control. They are NOT inexpensive, though (surprise, right?
). Carnival, on the opposite end, constantly has norovirus outbreaks. Most lines are somewhere in the middle.