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The number of people hit with a gastrointestinal illness on a Royal Caribbean International cruise ship has swelled to nearly 500, a company spokeswoman said Friday.

The increase to 475 people comes a day after the company said 277 passengers on the Oasis of the Seas had initially fallen ill.

Travelers on the cruise ship — which according to Royal Caribbean’s website is a 1,187-foot long vessel and was voted the best cruise ship 2014 by Travel Weekly readers — started feeling sick after the ship departed from Port Canaveral in Brevard County, Florida, on Sunday.

The cruise liner was scheduled to sail through the Caribbean before returning to Florida on Sunday. The Oasis of the Seas will now arrive back at Port Canaveral on Saturday. All of the ship’s passengers will get a refund due to the outbreak.

Passenger Shawn Popeleski told the Orlando Sentinel that they were told via a shipwide announcement Friday that the outbreak is “most likely” related to norovirus.

According to the Centers for Disease Control and Prevention, norovirus is a type of gastrointestinal illness and is “very contagious.” You can catch it from people infected by it and by touching contaminated surfaces or eating or drinking contaminated food and water.

Symptoms include stomach pains, nausea and diarrhea, according to the CDC. Although it is highly transmissible, the illness is “relatively infrequent on cruise ships,” the CDC states.