CHARLESTON, S.C. (AP) — About 350 people who got sick a week into a Caribbean cruise were responding well to medicine, the cruise line said Tuesday.

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 diarrhoea. Martinez says 27 of the nearly 850 crew members also reported symptoms.

The ship left Charleston on February 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. The trip was the first departure in a newly-expanded year-round schedule of cruises from South Carolina as the industry expands in the state. 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.