Experts predict that climate change will bring a number of perils to the planet, including rises in sea level and an increase in natural disasters. But could one perk of a warming world be a decrease in the spread of flu?

The short answer is: we don't know yet.

Studies in animals have shown that flu, or influenza, spreads less easily in air that is warm and humid. And it's even possible the mild weather most of the U.S. experienced this winter contributed, in part, to the so-far mild flu season.

But many factors play into flu transmission rates, including the population's level of immunity to the virus and the ways we interact, so it's impossible to tell at this point what effect climate change will have, experts say.

Why more flu in winter?

For years, researchers have searched for an answer as to why more cases of flu happen in winter. (In the United States, flu season typically occurs from November through March, with a peak in January and February.)

We still don't know exactly why, but researchers have some ideas.

One is that colder, drier air allows the virus particles to remain in the air for longer periods of time, and travel longer distances, said Christopher Olsen, a professor of public health at the University of Wisconsin-Madison.

Another contributing factor may be that, in winter, people tend to stay indoors, crowding together in ways that may increase opportunities for transmission.

Schools also start new sessions in September, which may allow for spreading of the virus among kids.

“Not only do you see temperature changes and humidity changes [in winter], you also see changes in people's social interactions,” said Dr. Bruce Lee, an associate professor at the University of Pittsburgh Graduate School of Public Health. This makes it difficult to determine which factors are responsible for flu's increased prevalence in the winter months, Lee said.

Mild winter, mild flu

Flu activity was so low in October 2011 through January 2012 that flu season did not officially begin until February, according to the Centers for Disease Control and Prevention. That's the latest start to the flu season in 24 years.

We also had a particular mild winter: the temperatures in March were the warmest on record, according to the National Oceanic and Atmospheric Administration.

But just because a mild winter and flu season occurred at the same time does not mean they are related, Olsen said.

There's so much variation in temperatures from year to year, and influenza activity from year to year, “it's really dangerous to try to make a connection” based on temperatures in one year. “You have to look at trends over decades,” Olsen said.

Higher flu vaccination rates and better hygiene practices due to increased awareness about flu could also be responsible for this year's mild flu season, Lee said.

In any case, flu season is unpredictable, so we could still see an uptick in activity. The H1N1 “swine flu” outbreak of 2009 began in April.

More research on flu transmission in tropical regions, as well as analysis of how flu activity relates to weather patterns, is needed to help us understand how climate change will affect flu transmission, Olsen said.

Pass it on:  Climate change may influence flu transmission, but it's hard to tell how it will be affected in the coming years.

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