For the third cruise in a row, some of the passengers aboard Celebrity Mercury have fallen ill, apparently with the norovirus.
The ship departed Charleston, S.C., last week. Then 350 of the 1,829 passengers aboard became sick, a spokeswoman for the cruise line told The Associated Press.
The crew onboard is "conducting enhanced cleaning, to help prevent the spread of the illness" and will return to Charleston a day early to "conduct an extensive and thorough sanitizing onboard the ship and within the cruise terminal," Celebrity Cruises said in a statement.
More than 14 million people are expected to take a cruise in 2010, according to the Cruise Lines International Association. So how can you help prevent any illnesses on your next cruise?
ABC News senior health and medical editor Dr. Richard Besser stopped by "Good Morning America" to explain how to stay healthy on the high seas.
The norovirus is highly contagious and its symptoms include nausea, vomiting, diarrhea, cramps and chills, Besser said.
The illness typically lasts 12 to 72 hours and usually occurs in places where people are in close quarters, such as schools, hospitals, nursing homes and cruise ships, according to Besser. It is the leading cause of foodborne outbreaks of diarrheal disease, accounting for more than 50 percent of these outbreaks.
It spreads through food, water, contaminated surfaces and person-to-person contact, Besser said. He added that the reason it could be so hard to get rid of on a cruise ship is because people can also bring it on board and it can live on surfaces for more than a week.
Cruise ships often visit places that have poor sanitation, so passengers can easily catch it from a restaurant they visited at a port, Besser explained.
Additionally, the infection dose of norovirus is very low, Besser said. It takes only 10 particles of the virus to make you stick, as opposed to the 100,000 particles of salmonella you would have to be exposed to in order to get sick.
Therefore, even if the crew does a very good job of cleaning the ship it only takes a little of the virus to get people sick again.
Before you book a cruise, check the ship's inspection score at the Centers for Disease Control and Prevention's Web site. While this won't necessarily protect you from norovirus, it provides a good indication of overall ship hygiene.
The CDC gives all of the cruise ships a score for cleanliness and it reports these outbreaks through its vessel sanitation program.
Look to see if there have been recent norovirus outbreaks on your ship. A recent outbraeak of norovirus may increase the likelihood of another outbreak soon after, Besser said.
Also, it is an easy and basic step, but very important to help stave off norovirus: Wash your hands well with soap and water. Besser said using an alcohol-based hand gel will also help.
Be very careful when caring for someone who is ill with norovirus since it is so highly contagious. Bodily fluids can transmit the infection.
Carrie Finley-Bajak, president of Cruise Buzz from the travel agency Cruise Holidays, told ABC News last month that passengers should bring disposable disinfecting wipes to wipe down doorknobs, phones and other surfaces in the cabin.