updated 3/4/2010 9:46:23 AM ET 2010-03-04T14:46:23

It's one of the hallmarks of winter: The misery of being stuck in bed with the flu. Now, scientists are finally figuring out why the virus hits hardest in the wintertime and why some winters are worse than others.

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Blame dry air.

Extremely low humidity levels in winter, according to new research, fuel influenza outbreaks. Particularly dry spells make the problem worse. The discovery might help scientists prepare for epidemics and for the rash of secondary illnesses, like pneumonia, that often slam people once they're already down.

"It is the first step toward potentially forecasting the risk of influenza outbreaks," said Jeffrey Shaman, an atmospheric scientist at Oregon State University in Corvallis. "By getting a handle on what's going on with influenza, we are also getting a handle on the other diseases that really piggy back on influenza."

To explain why flu and related illnesses strike far more often in the winter than at other times of year, theories have fallen into three categories. One idea is that people spend more time indoors in the winter and schools are in session, so there is more person-to-person contact.

Another theory is that, with less exposure to sunlight, people have lower levels of melatonin and vitamin D, weakening their immune systems and making them more likely to succumb to influenza viruses. Scientists have also hypothesized that temperature and humidity affect how long the virus can last after someone coughs or sneezes.

Previous research has shown that influenza viruses survive longer in air when temperatures are colder and relative humidity is lower. Relative humidity, which appears in many weather reports, describes how close conditions are to the point of forming fog or clouds.

But relative humidity isn't the best measurement for studying flu outbreaks, Shaman said, because relative humidity varies with temperature. So, there is actually less moisture in the air on a rainy winter day in Seattle than there is on a sunny summer day in the same city.

He thought it would be more useful to look at absolute humidity, which measures exactly how much moisture is in the air, regardless of temperature.

On that scale, Shaman said, winters are usually twice as dry as summers in a place like San Diego and Arizona, four times drier in New York, and up to five times drier in a particularly cold state like Minnesota.

Along with colleagues, he analyzed 31 years of data from around the United States and used a computer model to show that influenza outbreaks were more likely to occur when absolute humidity levels were low. Like a sliding scale, progressively drier air led to progressively higher likelihood that an outbreak would occur, the researchers reported in the journal PLoS Biology. Temperature didn't play much of a role.

"People had recognized that there was seasonality to this, but nobody has really come up with a unifying explanation," said Gregory Poland, Director of the Mayo Clinic's Vaccine Research Group in Rochester, Minn. Humidity, he said, "is likely is part of that unifying explanation."

Humidity is probably not the only explanation, however, and the weather forecast will probably never serve as a flu forecast. Even in dry conditions, the virus needs to be hanging around, and people need to come into contact to spread it. Still, any insight into what drives epidemics is a step toward saving lives.

When absolute humidity is low, for example, local hospitals could start stockpiling anti-viral medications and other supplies, and they could increase moisture levels in patient rooms.

Humidifying your home could help, too, but moist air alone is not the answer, Shaman said.

"The best defense against influenza remains vaccination. That can't be stressed enough," he said. "I would never suggest anyone forgo that to go out and buy a humidifier."

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