Gourmet food, first-class service, Las Vegas-type shows and a little offshore gambling… that’s what you’d expect on a vacation cruise.

NOT severe diarrhea.

But cases of diarrheal disease in passengers on cruise ships have increased since 2001, despite favorable results from health inspections of the ships.

What’s the cause? New technology is positively identifying biological stowaways called noroviruses as the culprit. So say experts at the Centers for Disease Control and Prevention. Noroviruses affect the stomach and intestines, causing gastroenteritis or inflammation of the gastrointestinal tract. Symptoms include vomiting, diarrhea and stomach cramping that can last from twelve to sixty hours.

These viruses spread by consuming contaminated food or liquids or by touching contaminated objects or surfaces and putting hands near or in the mouth. Direct contact with an infected person also can transmit infection.

A recent study in the American Journal of Preventive Medicine showed that on an average seven-day cruise, three passengers were likely to come down with gastroenteritis. This compared with two cases per cruise in the year before the study.

Researchers suspect people are coming on board with the virus. In the ship’s confined quarters, passengers then can unknowingly deposit the virus on various surfaces where others can easily be infected.

The most effective preventive measure is frequent and thorough hand-washing. Researchers also called on the cruise industry to heighten vigilance for gastroenteritis among passengers and crew.

Ship to shore, these steps can go a long way toward smooth sailing.