Why is flu spread in the winter?

A New York Times article suggests that flu spreads more easily in the winter because the flu virus is more stable in colder, dryer conditions. See:

Study Shows Why the Flu Likes Winter
By GINA KOLATA
Published: December 5, 2007
New York Times: Health

Researchers in New York believe they have solved one of the great mysteries of the flu: Why does the infection spread primarily in the winter months?

The answer, they say, has to do with the virus itself. It is more stable and stays in the air longer when air is cold and dry, the exact conditions for much of the flu season. (cont.)