CHARLESTON, S.C. - About 350 people who got sick a week into a Caribbean cruise were responding well to medicine, the cruise line said yesterday.

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. She said 27 of the nearly 850 crew members also reported symptoms.

The ship left Charleston Feb. 15. State officials said there has been an outbreak in norovirus cases across South Carolina but that it is not possible to say whether that's what led to the illnesses at sea.

Martinez said 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.

Norovirus is often to blame for similar symptoms sweeping closed quarters like those on cruise ships, but a determination will have to wait until samples are tested. Samples from ill passengers and crew are being sent to the Centers for Disease Control, said CDC spokesman Jay Dempsey.

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.

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 is the virus, it might spread quickly aboard ship because the virus stays on surfaces like doorknobs, handrails and sink fixtures for a long time, he said.

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