Celebrity Cruise spokeswoman Cynthia Martinez said Friday that 55 of the nearly 1,900 passengers on board the Celebrity Mercury were sick.
More than 400 passengers and crew fell ill during a cruise that ended in Charleston on Feb. 26. The vessel sailed again last weekend after a one-day delay to allow a crew to sanitize the ship.
The Centers for Disease Control and Prevention said tests showed last month's outbreak was caused by norovirus, which can spread quickly in closed quarters. Symptoms include nausea, vomiting, diarrhea and stomach cramps.
South Carolina health officials have reported twice as many cases of norovirus as normal this winter.
The virus may have come on board the ship with passengers, crew members or supplies. But it's almost impossible to pinpoint a specific cause in a closed place like a cruise liner, said Adam Myrick, a spokesman for the state Department of Health and Environmental Control.
"It's so ubiquitous. It's everywhere," he said. "There's no way you can say it came from this stairwell, this handrail in this stairwell."
And because the virus lingers in those who have been ill even after they feel better, even extensive cleaning can make it hard to prevent outbreaks.
"It's very difficult to do because you can shed the virus up to two weeks after a patient doesn't have the symptoms anymore," he said. "As soon as someone sets foot into a supposedly sanitized environment, they can still be shedding the virus."
The CDC Vessel Sanitation Web site listed three other ships scheduled to return from cruises Friday with reports of intestinal illnesses, one confirmed as norovirus.
The Celebrity Cruise Millennium sailing from Puerto Rico reported 180 of 2,958 sick in illnesses confirmed to be norovirus.
A total of 116 of the 3,006 passengers and crew aboard the Royal Caribbean International Jewel of the Seas took ill on a cruise out of Miami while 287 of the 1,770 aboard the Holland American Line Maasdam out of Fort Lauderdale were sick. The CDC said the causes of those outbreaks have not been confirmed.
For all of 2009, the CDC site listed a total of 15 outbreaks of intestinal illnesses on cruise ships. This year there have been seven, about on pace with last year when, by this date, there were six.
Two outbreaks last year on the Mercury were blamed on norovirus. An estimated 14.3 million passengers are expected to take cruises this year, according to the Cruise Lines International Association, an industry trade group.
The Mercury sailing last month ushered in Charleston's first year-round cruising season. There will be 67 cruise calls in the city this year. In the past, there had been only a handful of winter cruises.
Later this spring, Carnival Cruise Lines will permanently base its 2,056-passenger Carnival Fantasy in the city.