Officials say the number of passengers believed to be sick with the highly contagious norovirus has nearly doubled aboard a cruise ship sailing to ports in California.

Princess Cruises spokeswoman Karen Candy said on Wednesday that 66 passengers and 17 crew members were sick and had to be isolated to their cabins aboard the Crown Princess. That’s up from about 37 passengers who reported being sick while the ship was in San Francisco on Monday.

Officials believe norovirus is to blame for the illnesses. It is spread person-to-person and can include nausea and diarrhea.

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Candy says the ship’s staff has disinfected door handles and other surfaces.

The Crown Princess is on a seven-day cruise that includes stops in San Diego and Santa Barbara. It ends in Los Angeles on Saturday.

Photo Credit: Some 66 passengers and 17 crew members were sick and had to be isolated to their cabins aboard the Crown Princess April 9, 2014. Pictured, the Crown Princess travels into Port Canaveral, Florida in this July 18, 2006 file photo. T Joe Skipper / Reuters