People need to learn how to wash their hands.
Lacking to do so is what causes the viruses on cruise ships. In the isolated environment of a ship a norovirus can spread very rapidly. The Montezuma's-Revenge-like stomach problems can be severe to the very young, very old, and the very weak. Otherwise, symptoms persist for a day or two.
The risk factor for getting sick on a cruise will remain the same until the passengers are taught how to properly clean themselves.