now to say what the source is," said David Forney, chief of the cruise ship sanitation program at the national Centers for Disease Control and Prevention. He said it's likely passengers brought the virus with them when they boarded and it spread from there, but U.S. inspectors will look at whether food or water contributed to the outbreak.

The cruise industry, which is required to report the illnesses to the CDC, records dozens of viral outbreaks annually, with most affecting fewer than 100 people.

While often publicized, the cruise ship outbreaks are far less common than they seem, affecting about one in every 3,600 cruise ship passengers, said Christine Fischer, spokeswoman for the International Council of Cruise Lines, an industry lobbying group.

"Your chances of actually contracting norovirus are much higher on land than on a cruise," she said.

As of Tuesday, 556 guests and 154 crewmembers on board the Liberty had reported symptoms lasting one to three days, according to a statement issued Wednesday by Carnival Cruise Lines. The Miami-based company said the ship is carrying 2,804 paying passengers and 1,166 crewmembers.

The ship, currently on a 16-day tour that began Nov. 3 in Rome, is scheduled to arrive in Fort Lauderdale on Sunday.

The outbreak began within 24 hours of the departure and raged the strongest during the first five days while the ship was crossing the Atlantic Ocean, Forney said.

Under CDC supervision, the ship's crew launched a program of cleaning equipment, isolating sick passengers and other steps aimed at halting the bug, which often spreads via hand-to-hand contact on contaminated surfaces, Forney said.

Since Monday, the number of passengers reporting symptoms fell dramatically, he said, suggesting that the ship is getting the outbreak under control. Nobody had to be removed from the ship, Carnival said.

"It appears the interventions are starting to work," Forney said. "Within 24 hours of sailing, they had a lot of people sick. It has tapered off considerably over the past couple days."

When the ship reaches St. Maarten today the CDC will put three environmental health specialists on the ship to oversee the cleaning operation and lead the hunt for the cause of the outbreak, Forney said.

The ship was originally scheduled to set sail again Sunday, but Carnival pushed the departure day back to next Tuesday so that the crew could have two days to disinfect the ship, Carnival officials said.

The CDC could push Carnival to keep the ship in Fort Lauderdale longer if investigators find that more cleaning is necessary.

The CDC last inspected the ship March 4 and gave it a 94. Scores below 85 are considered unacceptable.

In cases of outbreaks, the CDC conducts investigations if more than 3 percent of passengers or crew come down sick.

The Liberty, which had its maiden voyage in July 2005, is one of the world's largest cruise ships, with 13 passenger decks and space for 2,974 travelers.

Staff Writer Nancy McVicar and Researcher William Lucey contributed to this report.

Sofia Santana can be reached at svsantana@sun-sentinel.com or 954-356-4631.