(CNN) -- Like a combination of disinfectant and vomit.
That's how a passenger on board a Royal Caribbean cruise described the ship's smell after hundreds of people fell ill.
Shannon Blace went on the trip with her fiance and 10 other friends and family members.
Of the 12, nine got sick, she said. One of the women in her group got sick twice.
In total, 595 of the 3,050 passengers and 50 of 1,165 crew members have become ill, according to Bernadette Burden, a spokeswoman with the Centers for Disease Control and Prevention.
They reported vomiting and diarrhea.
The cause of the illness is not clear, though the symptoms are consistent with norovirus, the cruise line said.
Noroviruses spread easily and are a common cause of gastroenteritis, which produces vomiting and diarrhea.
Because of the outbreak, Royal Caribbean cut its 10-day cruise short.
Explorer of the Seas left from Cape Liberty, New Jersey. It will return Wednesday, two days early.
"The number of reported new cases of gastrointestinal illness has dropped sharply after a spike in the first days of the cruise, and most guests who fell ill are up and about," the cruise line said in a Monday statement.
"The drop in new cases is encouraging. However, it is not unusual in an outbreak to still have smaller, secondary spikes. That is why, after discussions with the U.S. Centers for Disease Control and Prevention and our medical team, we decided the most prudent course for the health of our guests and crew was to bring the cruise home on Wednesday, two days earlier than planned."
The company said all passengers would get a 50% refund of their fare. All guests will also get a 50% future cruise credit.
Explorer of the Seas departed Cape Liberty on January 21.
It was scheduled to call to Labadee, Haiti; San Juan, Puerto Rico; Charlotte Amalie, St. Thomas; and Philipsburg, St. Maarten. It missed her port calls in Haiti and St. Maarten.
CNN's Miriam Falco contributed to this report.