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Federal health officials will stay on board a Royal Caribbean cruise ship until it gets back to port in New Jersey later this week, keeping an eye on patients and trying to find the source of a nasty stomach bug that’s sickened more than 620 people.

The Centers for Disease Control and Prevention says 577 passengers and 49 crew members are sick, and the most likely culprit is norovirus. The nasty stomach bug is one of the most common causes of illness in the United States and it spreads very quickly and easily — especially in close quarters like on a cruise ship.

“Our team is on the ship and will stay on board until it returns to New Jersey,” said Bernadette Burden, a spokeswoman for the CDC.

The Explorer of the Seas ship stopped part way through a 10-day cruise from Cape Liberty, N.J., to the Caribbean island of St. Maarten after patients started getting sick, with vomiting and diarrhea — classic symptoms of norovirus.

Cruise officials decided to cut the trip short. “After returning to home port on Wednesday, Jan. 29, we will perform a thorough ‘barrier’ sanitization program on the entire ship to make certain that any remaining traces of the illness are eliminated,” the company said in a statement.

“It will be the third aggressive sanitizing procedure the ship has undertaken since we became aware of the issue, and will additionally provide a window of more than 24 hours where there are no persons aboard the ship, which is a significant help.”

In the meantime, passengers with symptoms will be urged to stay in their cabins, where they’ll get regular checks, Burden said.

It’s a very common virus, Burden said. “The threat of norovirus is just as common on land as it is at sea,” she said.

It might be more noticed on a cruise ship because the patients are all in one place, together, for an extended time. An outbreak that starts in a restaurant might not be noticed by anyone because people will have dispersed and might not even know where they first got infected.

The virus is very easy to catch and very hard to kill. CDC says.

It can make as many as 21 million people sick every year and put 50,000 to 70,000 into the hospital. But it’s not especially deadly, killing 570-800 people every year. It spreads via what health officials call fecal-oral contact. People get it in their stool, and it can be found in feces before the patient even feels sick.

It’s one of the main reasons to wash hands thoroughly after using the restroom — both to protect yourself and to prevent spread. Anywhere that people gather, serve and share food or drinks are great places for the virus, because infected people can spread it by touching an object that’s someone else then touches or brings to their mouth.

The only real treatment is rest and fluids.

It’s not easy to clean up, either. CDC recommends using chlorine bleach or another certified disinfectant.