The New York Times ran a great article about a study that answers an age old question. Why does the flu (influenza) season always happen in the winter? We seldom see flu in Summer or even the Fall. In the tropics flu doesn't even exist.
There have been hypothesis that it was from overcrowding in schools and homes, or in the winter people are stuck inside and can pass the germs. But people are crowded in the Summer too and travel on vacations in planes and we don't see influenza.
A clever researcher, Dr. Palese from Mt. Sinai School of Medicine in New York conducted studies on guinea pigs and found that the flu virus thrives and is passed through the air at low temperatures. At 41˚the animals passed the virus more readily and longer than at 68˚. The virus was passed at low humidity but not at all at 80% humidity.
Since you really can't do anything about the weather, the best way to prevent flu is to get a flu shot or move to the tropics. Mom was almost right when she said "Put on that coat and gloves or you'll catch your death". It is amazing that it took this long to get some scientific evidence about a virus that is so common.