Of the 14.19 million cruise ship passengers whose vessels passed through at least one US port in 2015, a total of 2570 reported a case of gastrointestinal infection, says  the US Centers​ for Disease Control and Prevention

The CDC requires reporting only if the number of infected passengers, plus crew, is three per cent or more of the total number on board. Smaller outbreaks go unreported but even so, the chance of spending part of your cruise with your head over a toilet from gastro is tiny. While the figure may be low, once a gastrointestinal bug gets on board, it tends to spread like butter on a hot waffle.

The main culprit is the norovirus​, a common and highly contagious bug that thrives in confined spaces with large human populations. The norovirus spreads quickly through infected food and by hand-to-mouth contact.

Even something as simple as grabbing a handrail on your way to the restaurant or passing the salt shaker to a fellow diner can spread the infection. If your vessel detects an outbreak, avoid buffet-style dining in favour of a la carte, but most important of all is keeping your hands clean. Don't put anything in your mouth without first washing your hands and use the hand sanitiser that most cruise vessels carry in their restaurants.

If you do fall victim see the ship's doctor and drink plenty of water and electrolytes, since dehydration is a side-effect. There is no cure or vaccine. Symptoms will typically last one to two days but victims may be contagious for two weeks.