Reporter: For "Good morning America," Jeff Zeleny. And to the cruise ship coming home earlier this morning. Hundreds of passenger too sick to sail. John muller has the story. Reporter: This morning,... See More
Reporter: For "Good morning America," Jeff Zeleny. And to the cruise ship coming home earlier this morning. Hundreds of passenger too sick to sail. John muller has the story. Reporter: This morning, what some are calling a giant floating petri dish now waking its way back to the states. The royal caribbean "Explorer of the seas" reversing course after more than 600 people fell violently ill. The first cases reported less than 24 hour after the ship left New Jersey. They skipped Haiti and headed for Puerto Rico and St. Thomas. That's where officials from the CDC boarded it. It just felt like one of the worst stomach bugs I have ever had in my life. Reporter: The passenger speaking to us. I am completely disappointed. We were really, really looking forward to this all year. Reporter: Right now, the cause of the outbreak sun clear. Royal caribbean says the symptoms appear to be consistent with that of norovirus. The bug responsible for seven outbreaks on cruise ships last year. They have been scrubbing the entire step top to bottom, nonstop. Reporter: In a statement overnight, royal crib January said, we think the right thing to do is to bring our guests home early and use the extra time to sanitize the ship more thoroughly. And while they said they'll compensate guests, they warn additional cases are to be expected. For "Good morning America," John muller, ABC news, New York.
This transcript has been automatically generated and may not be 100% accurate.