Motion sickness
Seasickness can turn a relaxing cruise on the calmest seas into an endless cycle of nausea and trips to the bathroom. This cruise ship illness is difficult to treat once symptoms begin. If you know you are prone to sea- or motion sickness, talk to your doctor before setting sail. A commonly used seasickness medication is scopolamine, a prescribed patch worn behind the ear that releases medication for 72 hours. Your provider may also suggest an anti-nausea medication or herbal remedy.

To keep seasickness at bay, avoid tasks that keep your head static and still, such as reading, knitting or computer work. If you develop nausea – the first sign of seasickness – go to an exterior deck and fix your gaze on a land-based object or the horizon. Lie face up if needed and keep yourself hydrated. Stay far from the open bar and dessert buffet table because alcohol and heavy meals can slow your acclimation to life at sea.

Respiratory illnesses
Your cruise ship will sail into the open seas, where circulating trade winds keep the air clean and fresh. However, respiratory illnesses are still potentially serious risks on cruise ships. Influenza and Legionnaires' disease can be easily passed among people living in close, contained quarters. Avoid close contact with obviously ill people on your cruise and if you are sick, protect others by notifying the ship's doctor, heeding her instructions and sticking to your cabin.

Tropical diseases
If your cruise ship calls on ports in Central or South America, Southeast Asia or the South Pacific, you might be at risk of exposure to malaria, dengue fever or yellow fever. The Centers for Disease Control and Prevention keeps careful watch on disease outbreaks around the world. Before your trip, check the listings at the CDC for the latest news on outbreaks in your destination. Bring a strong bug repellent and cover-all clothing to ward off bug bites in port. Malaria-carrying mosquitoes are most active at dawn, dusk and night, and tend to linger near the ground. Always wear pants that cover your ankles. Consult with your physician about your potential risk of exposure.

Also of interest: Does size matter? How to choose the best cruise ship.