The 10-day Caribbean voyage of the Explorer of the Seas was cut short Sunday night, because 600 passengers and about 40 crew had fallen ill. It never made it to its final destination, St. Maarten.
A team from the Centers for Disease Control and Prevention went aboard the ship when it docked at St. Thomas, U.S. Virgin Islands. The team is investigating the cause, and will sgtay on board the ship until it returns to Bayonne, New Jersey later this week.
Although the tests haven't been completed, health officials suspect the highly contagious norovirus is the cause.
The virus got its name from Norwalk, Connecticut, where the virus was formally identified, however, doctors say it's been around forever.
There are about 200 different strains and they vary from season to season.
But Dr. Neil Fishman, a Penn Medicine infectious disease specialist, says, "It only take about 18 virus particles to infect somebody. But when people have norovirus, they can shed somewhere between 70 and 100 BILLION particles."
And that is one reason it can spread so fast and can be hard to stop.
"It has almost no warning (small laugh). There was high fever, chills, aches, dehydration," says Arnee Dodd, one of the first ones to get sick on the ship.
By the next morning, the infirmary on the Explorer of the Seas was packed.
Dodd recalls, "The nurse walked out and looked at everyone and said if you're not sick, you have to leave right now, because this is spreading faster than we can contain it."
Norovirus is the most common stomach bug around, and it is the most common cause of foodborne illnesses.
But it also spreads through contaminated surfaces, water, even ice.
It is extremely hardy, surviving temperature from freezing to 140 degrees.
Dr. Fishman says norovirus isn't just a cruise ship problem, but spreads in other close places, such as schools and nursing homes.
We tend to be in more of those places in winter time, so the outbreaks are more common then, although they can occur anytime.
Royal Caribbean says the ship is repeatedly being cleaned and disinfected from top to bottom.
Sal Panto, the mayor of Easton, Pennsylvania, who was a passenger, saw that first-hand.
"I mean, they're doing the walls, the lounge chairs, the railings, the buttons to the elevators, the doorknobs to your room," he says.
But because norovirus is so hardy, it usually takes a bleach solution to completely wipe it out.
The best way to prevent getting sick in the first place is good hand hygiene.
"In this case, it's probably a little better to use soap and water, than an alcohol hand sanitizer because the physical activity of washing with soap and water gets the particles off," says Dr. Fishman.
We say it all the time, but it's true - washing your hands frequently and thoroughly does help protect from the norovirus, and many other illnesses.
Also, if you are sick with this, DON'T prepare foods or care for others.
And once you feel better, you still have to be very careful, because you still be shedding the virus for several weeks.
So you may still be very contagious even though you are feeling well.