Flu virus trots globe during off season( The influenza A virus does not lie d...),Health

The influenza A virus does not lie dormant during summer but migrates globally and mixes with other viral strains before returning to the Northern Hemisphere as a genetically different virus, according to biologists who say the finding settles a key debate on what the virus does during the summer off season when it is not infecting people.

"Nobody really knows why flu is a winter disease in the temperate regions and more continuous in the tropics," says Edward Holmes, professor of biology at Penn State. "The big question is, Why is flu seasonal""

Flu infections in the Northern Hemisphere typically follow a familiar pattern. Some time before the start of the winter infection season, the virus evolves, changing enough to evade the previously primed immune system. Then, just before summer, the virus disappears, only to resurface the next fall with a completely different genetic makeup, ready to fool the immune system anew.

But little is known about what happens to the virus between two successive winters, or how and where it is able to sustain itself.

The key question, Holmes explains, is whether the virus settles into a dormant state waiting for the right cues of temperature and sunlight to reactivate, or whether it migrates to viral reservoirs in the tropics, from where it is later reintroduced.

It is thought that places in Southeast Asia, where humans and animals live in close proximity, might be the permanent melting pot where viruses continually circulate and exchange genetic information.

To test the migration theory, Holmes and Martha Nelson, graduate student at Penn State, and their colleagues at the National Institutes of Health, Lone Simonsen, Cecile Viboud, and Mark A. Miller, analyzed the influenza A virus genomes of 900 virus samples from New Zealand, Australia and New York state dating between 1998-2005.

Their findings, outlined in this month's issue of PLoS Pathogens, reveal that the genomes of 52 viruses from Ne
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