Celebrity Cruise spokeswoman Cynthia Martinez said 326 of the more than 1,800 passengers on the Celebrity Mercury began complaining Sunday of upset stomachs, vomiting and diarrhea. Martinez says 27 of the nearly 850 crew members also reported symptoms.
The ship left Charleston on Feb. 15. State officials said there has been an outbreak in norovirus cases across South Carolina but that it is not possible to say if that's what led to the ship's illnesses.
Martinez says the crew is conducting "enhanced cleaning" of the ship to prevent the spread of the illness.
An extra doctor and two nurses came aboard in St. Kitts, in the Leeward Islands, and will sail to Charleston, arriving early Friday.
It's not clear what caused the outbreak. Norovirus is often to blame for similar symptoms sweeping closed quarters like those on cruise ships, but a determination will have to until samples are tested.
Samples from ill passengers and crew are being sent to the Centers for Disease Control, said CDC spokesman Jay Dempsey. He said workers from the agency's Vessel Sanitation Program will meet the ship when it arrives in Charleston.
The workers will conduct an environmental assessment of the ship to determine the cause of the illness, he added.
According to the CDC Web site, there were two outbreaks of norovirus, which causes stomach flu, last winter on the Celebrity Mercury. In all, the agency investigated 15 outbreaks of gastrointestinal illnesses on cruise ships calling at American ports.
This year an estimated 14.3 million passengers are expected to take cruises, according to the Cruise Lines International Association, an industry trade group.
The Mercury embarked from a state where health officials have reported twice as many cases of norovirus as normal this winter.
"We have been taken aback at how many people are getting sick with this virus," said Adam Myrick, a spokesman for the South Carolina Department of Health and Environmental Control.
If it turns out to be the virus, it's no surprise it spread quickly aboard ship because the virus stays on surfaces like doorknobs, handrails and sink fixtures for a long time, he said.
"Any time you have tight spaces and shared surfaces, this virus can spread fast," Myrick said.
The South Carolina cruise industry is growing and the Mercury sailing earlier this month began Charleston's first year-round cruising season. There will be 67 cruise calls in the city this year.
The Celebrity Mercury has six more departures set from Charleston during the coming months, including a 16-night trip through the Panama Canal ending in Los Angeles.
Later this spring, Carnival Cruise Lines will permanently base its 2,056-passenger Carnival Fantasy in Charleston.
As the industry grows, the South Carolina State Ports Authority is pursuing plans to open a new cruise terminal and open another half-mile of Charleston's historic waterfront to the public.
A recent study commissioned by the authority shows cruises will mean $37 million to the South Carolina economy this year.