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(NBC News) -- A nasty new strain of norovirus, a highly contagious gut bug, has circled the globe and landed in the U.S., where it's now the leading cause of what's known indelicately as "winter vomiting disease."

Health officials at the Centers for Disease Control and Prevention said Thursday that the GII.4 Sydney strain of norovirus was responsible for more than half of outbreaks of the illness during the last four months of 2012. The new norovirus spread amid a particularly harsh flu season that's also causing misery.

Of 266 outbreaks of norovirus between September and December, 141 were caused by the bug that was first detected in Australia in March 2012, according to data from CaliciNet, which tracks norovirus outbreaks. The proportion of outbreaks caused by the new strain jumped dramatically from 19 percent in September to 58 percent in December, the CDC says in its weekly report on death and disease.

"Right now, it's too soon to tell whether the new strain of norovirus will lead to more outbreaks than in previous years. However, CDC continues to work with state partners to watch this closely and see if the strain is associated with more severe illness," said Dr. Aron Hall, a CDC epidemiologist specializing in viruses.

Like the virus that causes the flu, norovirus mutates quickly, resulting in a new strain every few years, Schaffner said. When that happens, people who've already had previous versions of the bug that can cause profuse vomiting and diarrhea are more likely to get it again."What that means is more of us are susceptible," said Dr. William Schaffner, a infectious disease expert at Vanderbilt University in Nashville, Tenn.

That can be a particular problem in enclosed spaces, such as cruise ships, nursing homes or other gatherings where people share close quarters.

The virus spreads ridiculously easily, often carried in the air after projectile vomiting, or lingering on surfaces where it infects the next victim, Schaffner said.

"It's very contagious," he said. "It takes only a few viral particles."

In the U.S., norovirus is the leading cause of acute gastroenteritis, affecting more than 21 million people a year and leading to about 800 deaths, CDC said.

Young children and the elderly are most at risk of serious complications, typically because of the danger of dehydration from rapid fluid loss.

Another danger is that dehydration can cause blood pressure to drop, resulting in fainting that can lead to falls. Schaffner said when he heard that U.S. Secretary of State Hillary Clinton suffered a concussion after fainting following a stomach virus in December, his first thought was norovirus.

"She's a global traveler; she could have picked it up anywhere," he said.

CDC officials advised health workers to be vigilant for increases in norovirus outbreaks this winter because of the new GII.4 Sydney strain. They should follow standard infection control practices to prevent norovirus.

In addition, the general public ought to be aware that the new bug is out there and take precautions including washing hands with soap and water, disinfecting surfaces, rinsing fruits and vegetables, cooking shellfish thoroughly and not preparing food or caring for others while ill.

CDC officials said it's too early to tell whether the new strain will lead to more outbreaks or more serious illness, but they're watching the situation closely.

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