Update: MarketWatch reports that more than 600 passengers on Explorer of the Seas are ill, and the ship is returning to New Jersey two days early.

The CDC says 577 passengers, or 18.9%, reported being ill, as did 49 crew members, which is 4.2% of their total.

The CDC says the cause of the outbreak is “unknown,” although the government agency boarded the ship in St. Thomas on January 26 to conduct an epidemiologic investigation.

The original post follows:

The U.S. Centers for Disease Control and Prevention says 281 passengers have fallen ill aboard a Royal Caribbean cruise ship.

The CDC said in a statement Friday that another 22 crew members also are ill, with people reporting vomiting and diarrhea. Authorities said it’s not yet clear what is causing the illness.

The CDC said health officials will board the ship on Sunday while it’s docked in St. Thomas in the U.S. Virgin Islands. Authorities said specimens are being collected and will be tested.

A total of 3,050 passengers and 1,165 crew members are aboard the Explorer of the Seas.

The CDC said Royal Caribbean officials have taken action, including increasing disinfection procedures and preparing new crew members to join the ship mid-voyage.

Photo Credit: Royal Caribbean's Explorer of the Seas at Roseau in Dominica, as pictured in 2008.. Flickr.com