(CNN) -- Like a combination of disinfectant and vomit.
That's how a Royal Caribbean cruise passenger described the ship's smell after hundreds aboard fell ill.
Shannon Blace had boarded the ship in Cape Liberty, New Jersey on January 21 for with her fiance and 10 other friends and family members.
Of the 12, nine got sick, she said Monday night. One of the women in her group got sick twice.
By Tuesday morning, the number had risen to 10 when her fiance fell ill and was ordered quarantined in his room, she said.
It was supposed to have been a 10-day cruise. But the liner announced it was cutting it short by two days as a result of the outbreak.
The atmosphere in the ship's usually bustling promenade deck was subdued on Tuesday. "Everywhere you look, you see dreary sad faces," Blace said in a telephone interview. "Just unhappy people everywhere—waiting to get back to New Jersey."
In all, 622 of the 3,071 passengers and 50 of the 1,165 crew members became ill, according to the Centers for Disease Control and Prevention.
They reported vomiting and diarrhea.
A CDC vessel sanitation program epidemiologist, a contract epidemiologist and an environmental health officer boarded the ship in St. Thomas on the U.S. Virgin Islands to study the outbreak and the response on the voyage back to Cape Liberty.
Five specimens were sent Sunday to the CDC lab for testing.
The cause of the illness was 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.
"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 Monday in a Monday.
"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 and a 50% credit for a future cruise.
Those passengers who were ordered quarantined to their rooms will get an additional credit of one future cruise day for each day in confinement, it said.
After returning to home port on Wednesday, the ship will be cleaned "to make certain that any remaining traces of the illness are eliminated," the liner said in a statement.
"Guests scheduled for the next cruise on Explorer of the Seas can be confident that all possible measures will have been taken to prevent further problems," it added.
Explorer of the Seas departed Cape Liberty on January 21 for what was to have been a 10-day cruise.
It had been scheduled to call to Labadee, Haiti; San Juan, Puerto Rico; Charlotte Amalie, St. Thomas; and Philipsburg, St. Maarten. It missed port calls in Haiti and St. Maarten.
CNN's Tom Watkins and Miriam Falco contributed to this report.