Cruise ships are susceptible to outbreaks of norovirus and other communicable gastro-intestinal illnesses because of the large number of people sharing facilities, as well as the quick turnover that many ships encounter. These viruses are spread much like the common cold, so frequent hand-washing is the best preventative measure; many cruise ships now provide dispensers of hand disinfectant in their restaurants. If you become ill (relatively mild vomiting and diarrhea are typical symptoms of norovirus), the medical staff will probably ask you to remain in your cabin and cancel your disembarking privileges until after you recover, to avoid exposing other travelers to the virus.   Although shipboard food and water will be sanitary, the usual precautions for overseas travel should be taken when eating and drinking ashore. In very touristy areas the water may be perfectly safe to drink; in less-developed ports you may need to be careful to avoid local water- and food-borne bacteria.