(CNN) -- Some people sickened by an outbreak of a gastrointestinal illness on a U.S. cruise ship have recovered, but hundreds remain ill, the cruise company said Wednesday.
Celebrity Cruises ship Mercury, which departed Charleston, South Carolina, last week for the eastern Caribbean, has 326 ill passengers and 32 sick crew members, company spokeswoman Cynthia Martinez told CNN on Wednesday.
On Tuesday, 419 of the 1,838 passengers were sick, she said.
The cruise company had reported that 27 of the 849 crew members were ill on Tuesday.
Symptoms of the sickness included upset stomach, vomiting and diarrhea, the cruise company said earlier this week. The sickened passengers and crew have been administered over-the-counter medicine and are responding well, Celebrity Cruises said.
But it is still unclear how they became sick, Martinez said.
Medical samples were sent to a lab for testing, Martinez said, but said the company has not yet received the results.
She has said that guests who were in isolation while ill will receive compensation.
The ship's medical facility began treating guests Sunday, and by Monday, hundreds of others were sick, too, Celebrity Cruises reported.
A doctor and two nurses joined the cruise medical staff Monday to help with the overload of patients when the ship stopped in Tortola, British Virgin Islands, according to the cruise company. The ship will be at sea until it returns to Charleston on Friday, Martinez said.
To control the outbreak, the crew has stepped up cleaning of the ship, as the Centers for Disease Control and Prevention advises when an outbreak of gastrointestinal illness occurs.
Norovirus commonly causes viral gastrointestinal illness outbreaks on cruise ships, the CDC says. It can spread from contact with contaminated food or drink, by touching objects infected by people who are already sick, or through close contact with people who are infected, according to the CDC.
So far this year, three gastrointestinal illness outbreaks have occurred on cruise ships that docked at U.S. ports, according to the CDC. Norovirus was the cause of two outbreaks on the Mercury in 2009, the CDC reported.
The outbreaks reported and investigated by the CDC infected at least 3 percent of the people aboard cruise ships carrying at least 100 passengers on cruises lasting between three days and three weeks.