The ship Explorer of the Seas is on its way back to port after an outbreak of what the Centers for Disease Control and Prevention (CDC) says is the norovirus. The outbreak has sickened 600 people aboard the Royal Caribbean cruise.

Norovirus is a rapidly spreading stomach bug that became so bad onboard the ship, it literally forced the cruise line to cut the 10-day cruise short.

CDC officials, who have boarded the Explorer of the Seas to investigate the outbreak, say the likely culprit is norovirus.

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

The CDC says it is currently running tests to make sure they’ve identified the culprit, which spreads very easily in contained spaces such as cruise ships.

The cruise ship had 3,050 passengers aboard, with a crew of 1,165 people, and left Cape Liberty on Tuesday for a 10-day cruise through the U.S. Virgin Isles, and Puerto Rico, with stops along the way in ports such as San Juan, St. Thomas and St. Maarten.

The ship should have come back the following Friday, but due to the quickly spreading sickness, it is expected to arrive back in New Jersey on Wednesday.

“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,” Royal Caribbean said in a statement.

Adding, “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.”

Even with scrupulous cleaning, the virus can still remain intact, according to Dr. Richard Besser, ABC’s chief medical correspondent.

“The virus is not killed by alcohol sanitizers. It survives and can be infectious on surfaces,” he said. “People who are infected will shed the virus for days after their symptoms resolve. This is a real problem for food workers on cruise ships. Lastly, if you go into a bathroom that has been used by someone who has the norovirus, you can get sick.”

According to the CDC, norovirus causes about 21 million cases of acute gastroenteritis in the U.S. each year, hospitalizing between 51,000 and 71,000. It isn’t very deadly, however, resulting in fewer than 1,000 deaths each year.

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