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It has not been the best season for cruise ships. By the time the Explorer of the Seas docked at Bayonne, N.J., in late January, more than 600 passengers and crew members were sick to their stomachs. And the Caribbean Princess arrived in Houston the same day after an outbreak sickened at least 192 people onboard.

Over the past five years, an average of about 14 cruise ships a year have had outbreaks of diarrheal illness, and the culprit is almost always norovirus, as it was on these two ships.

So if you go on a cruise, are you putting yourself at risk for this illness? Well, not exactly, but the answer is complicated.

Norovirus infects 20 million Americans a year, according to the Centers for Disease Control and Prevention, thriving in closed areas like dormitories, summer camp cabins, health care facilities and other places in addition to cruise ships. It spreads through contaminated food or water or by contact with contaminated surfaces. In addition to loose stool and vomiting, it can cause weakness, muscle aches, headache and fever. There is no treatment, and most people recover in a few days.

The best way to avoid it is prevention, and the best prevention is hand washing. The agency recommends that cruise ship passengers wash before eating or any other action that involves bringing hands near the mouth. And it recommends washing your hands after using the toilet, changing a baby’s diaper and coming into contact with communal features like railings.

Bringing it aboard

Though cruise outbreaks make news, Jan Vinjé, head of the National Calicivirus Laboratory at the Centers for Disease Control and Prevention, said that widespread illness occurs on only about 1 in 200 voyages. The cause is not necessarily cruise line maintenance.

“The food served on ships is usually of excellent quality, and food preparers are well trained,” Vinjé said. And when illness appears, he added, crews clean quickly and effectively. The problem, he said, is passengers. “If Grandma is sick when she gets on, she’s going on the cruise anyway,” Vinjé said. “And that’s how the virus gets onboard. Then it lands on handrails and doorknobs, and the transmission continues.”

Dr. Philip C. Carling, a clinical professor of medicine at Boston University, said that regardless of the origin, once onboard, the illness spreads widely. He said the reason is failure to clean restrooms properly.

“Of course they’ve been doing a good job with food,” Carling said. “And if a person vomits, they soak everything in bleach. But they’re not doing any routine examination of cleaning processes.”

The CDC does inspect ships but not every changing table. “We inspect some bathrooms, and we don’t inspect for norovirus,” said Bernadette Burden, an agency spokeswoman.

Its inspection reports typically cite ships for inadequate chlorine in swimming pools, food stored at the wrong temperature, dirty cookware and other problems. In 2013, the CDC did 231 inspections and issued 17 failures — scores of 85 or less on a 100-point scale. The inspectors awarded perfect scores of 100 to 27 ships, and most others scored in the mid- or high 90s.

If it helps put your mind at ease on your next excursion, bear in mind that cruise lines do their best to avoid outbreaks, and the unflattering attention that they can bring. Michael McGarry, senior vice president at Cruise Lines International Association, an industry trade group, said that cruise companies take careful steps to control illness.