CNN reports that hundreds of people aboard two cruise ships in the Caribbean fell ill due to norovirus, the latest instances in which the stomach bug has afflicted vacationers at sea. The U.S. Centers for Disease Control and Prevention reported Friday that norovirus had hit more than 600 passengers and crew on Royal Caribbean’s Explorer of the Seas, making it one of the biggest such outbreaks on any cruise ship in the past two decades. In addition, the Caribbean Princess, operated by Princess Cruises, cut short its own seven-day Caribbean itinerary on Friday. Princess Cruises said that 178 passengers and 11 crew members aboard had been stricken with norovirus, with CDC staffers on board Friday to help sanitize that ship. There have been nearly 200 confirmed norovirus outbreaks on cruise ships in the past 20 years. According to CDC, norovirus is the most common cause of acute gastroenteritis in the U.S. Each year, it causes 19-21 million illnesses and contributes to 56,000-71,000 hospitalizations and 570-800 deaths. Norovirus is also the most common cause of foodborne-disease outbreaks in the United States.