Centers for Disease Control and Prevention officials, who have boarded the Explorer of the Seas to investigate the outbreak, say the likely culprit is norovirus.
"Our team is on the ship and will stay on board until it returns to New Jersey," said CDC spokeswoman Bernadette Burden.
The CDC says it is running tests to make sure they've identified the culprit, which spreads very easily in contained spaces such as cruise ships.
The ship, carrying 3,050 passengers and 1,165 crew, departed Cape Liberty last Tuesday for a 10-day trip through the U.S. Virgin Islands and Puerto Rico, including stops at San Juan, St. Thomas and St. Maarten. After incidence of the illness spiked over the weekend, Royal Caribbean decided to return the ship to port early.
It is expected to arrive back in New Jersey on Wednesday.
“After returning to home port on Wednesday, Jan. 29, we will perform a thorough ‘barrier’ sanitization program on the entire ship to make certain that any remaining traces of the illness are eliminated,” Royal Caribbean said in a statement.
“It will be the third aggressive sanitizing procedure the ship has undertaken since we became aware of the issue, and will additionally provide a window of more than 24 hours where there are no persons aboard the ship, which is a significant help.”
According to the CDC, norovirus causes about 21 million cases of acute gastroenteritis in the U.S. each year, hospitalizing between 51,000 and 71,000. It isn't very deadly, however, resulting in fewer than 1,000 deaths each year.
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