Once again, a cruise ship is limping into port carrying hundreds of passengers and crew sickened by norovirus. But what exactly is norovirus, and why does it spread so easily on cruise ships?

Wednesday (Jan. 29), the Royal Caribbean cruise ship Explorer of the Seas returned to port in Bayonne, N.J., after a suspected outbreak of norovirus struck almost 600 of the vessel's passengers and crew, according to NBC News.

Norovirus consists of a group of viruses that cause gastrointestinal illness in humans (the viruses are not known to infect other animals). The norovirus causes about 20 million illnesses in the United States each year, according to the Centers for Disease Control and Prevention (CDC), and is blamed for as many as 800 U.S. deaths each year. [7 Devastating Infectious Diseases]

Norovirus is named for Norwalk, Ohio, where the first confirmed outbreak was recorded, in 1968. People sometimes refer to a norovirus infection as "stomach flu," even though the virus is not related to influenza.

How norovirus spreads

The notoriety of norovirus comes from the ease with which it spreads from one person to another: You can catch it by ingesting food or drink that's been contaminated, or by touching any contaminated surface, then touching your nose, mouth or eyes. The virus is also aerosolized, or sprayed into the air, when an infected person vomits or flushes a toilet, and can spread when a person inhales the virus.

Unlike many germs, norovirus isn't easily destroyed by soap and water, and it can survive on surfaces for at least a week, and in water for months. Alcohol doesn't have much of an effect on norovirus, either, rendering alcohol-based hand sanitizers relatively useless.

Bleach-based cleaning solutions will destroy norovirus, however, and washing with soap and water will remove the virus from your hands, but might not destroy it. The virus' sheer toughness and durability helps explain why it survives so well on cruise ships, where it can spread easily from one person to another.

But norovirus doesn't just haunt cruise ships. Outbreaks of the contagion have been reported in other places where large numbers of people share relatively close quarters, including nursing homes, hospitals, scout jamborees and even restaurants where an infected person vomited.

Symptoms of norovirus

Norovirus symptoms generally begin within a day or two of exposure. The effects include nausea, vomiting, diarrhea, and stomach pain and cramps. In some cases, victims also suffer fever, chills, headache, weight loss and fatigue.

Though these symptoms can be severe, they are usually short-lived, and most people recover within two days. Only particularly severe cases, usually involving young children, the elderly or people with compromised immune systems, require hospitalization.

Though the symptoms may end quickly, an infected person can continue to shed the virus, or infect other people with it, for up to three days after recovery, according to the CDC. This is another factor that makes a norovirus outbreak so tough to control.

Norovirus treatment and prevention

Because a norovirus infection usually resolves in a day or two, most cases don't require any extraordinary measures. And because it's a virus, you can't treat it with antibiotics.

Sports drinks and rehydration drinks can help replace fluids lost by diarrhea and vomiting. Though a vaccine for norovirus is in development, it has not yet been approved.

The best way to prevent a norovirus infection — on a cruise ship or elsewhere — is through careful handwashing and good general hygiene. Eat only foods that have been properly handled and prepared, avoid raw shellfish and other seafood, and do not drink untreated water.

If you have been infected with norovirus, do not prepare food for others for at least two days after you recover, the CDC recommends. Carefully wash any potentially contaminated laundry; clean toilets, other bathroom surfaces and all kitchen areas with a bleach-based solution.

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