The U.S. Centers for Disease Control and Prevention say 281 passengers have taken 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.