By Claire Zillman
January 30, 2014

FORTUNE — When the Royal Caribbean ship Explorer of the Seas returned to port in New Jersey on Wednesday, it delivered 700 sick passengers and crew, the highest number of ill people the Centers for Disease Control and Prevention has recorded on a ship in two decades.

Reports from passengers on the ship painted a grim scene: “Everybody was getting sick,” a passenger told News 12 New Jersey. “They had diarrhea. They were throwing up. It was just horrible.” Another passenger told CNN, “I never wanted to go home so much in my life.”

And that was just the latest in what’s been a steady stream of ships that have encountered similar outbreaks.

Earlier this month, a smaller outbreak that sickened 66 passengers occurred on another Royal Caribbean (RCL) vessel, the Majesty of the Seas. Last September, 335 people fell ill on a Celebrity Cruise Lines ship and, in 2012, a Princess Cruises ship had mass illness outbreaks in back-to-back months.

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With so much ickiness infecting these ships, surely a big fat finger should be pointed at the cruise lines for doing something wrong.

Actually, no, that’s not the case.

Most of the outbreaks have been caused by the norovirus, a highly contagious virus that “can infect anyone,” according to the CDC. It’s transmitted person-t0-person, and through contaminated food, water, and surfaces. The virus causes inflammation in the stomach, intestines, or both, and the most common symptoms are stomach pain, nausea, diarrhea, and vomiting. There is no sure-fire treatment. It’s a hearty virus that can withstand temperatures as high as 140 degrees Fahrenheit and can live on hard surfaces for as long as two weeks. It’s most prevalent in the winter months, from November through April, and it has been around “forever,” says Robert Kim-Farley, a doctor and professor in epidemiology at the UCLA Fielding School of Public Health.

The norovirus thrives in crowded living quarters, shared dining areas, and places where there’s a high turnover of people, according to Kim-Farley, and yes, cruise ships meet all three criteria. But short of abandoning their business model altogether, there’s little cruise lines can do to prevent norovirus outbreaks beyond the precautions they already take.

Royal Caribbean, for example, asks all guests boarding the ship if they’ve experienced any gastrointestinal symptoms within the last three days, according to company spokeswoman Cynthia Martinez. And if guests feel uncomfortable taking their cruise for reasons related to personal health or otherwise, the company lets them reschedule. Once onboard, passengers are bombarded with information about how to stay healthy — signs encouraging hand-washing after using the bathroom and before eating and complimentary sanitizing gel that’s located all over the ship. If guests become ill, the crew employs cleaning procedures with products strong enough to kill the norovirus. All cruise lines follow these kinds of procedures as required by the CDC’s Vessel Sanitation Program, which conducts often unannounced inspections of cruise ships. The VSP inspected Royal Caribbean’s Explorer of the Seas in July 2013 and scored it a 98 out of 100.

So what’s to blame for the ship outbreaks? Poor human hygiene — people not washing their hands, mainly — and nature, not necessarily the ships themselves.

“One of the issues is that when people are on a facility like a ship, you recognize when an outbreak is happening,” says Benjamin Lopman, an epidemiologist in the CDC’s division of viral diseases. “But the norovirus is really common. I’m not sure ships are particularly suspect.”

There are between 19 million and 21 million cases of norovirus each year in the U.S., says Kim-Farley, and an overwhelming majority of those cases occur on land, according to the CDC. Those cases lead to as many as 1.9 million outpatient visits, which result in 400,000 emergency visits and up to 71,000 hospitalizations. Between 570 and 800 people die from the norovirus each year, usually from dehydration.

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Norovirus outbreaks also often occur on college campus, in summer camps, and military barracks. A CDC spokeswoman said that one of the more notable outbreaks occurred years ago at an Atlantic City casino, where absolutely everything had to be sanitized, down to the poker chips and dice. One of the most common — and concerning — locales for an outbreak is nursing homes, says Lopman, where residents, who are vulnerable to begin with, live in close quarters and share public spaces.

Then again, nursing home residents often don’t choose to be there, while going on a cruise is entirely voluntary.

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