Why is there a cold and flu season? I was under the impression that the flu was carried by birds, but birds are around in the summer, so why do we get sick in the winter?
Christopher
Our immune system is less effective in cold weather.
Not True! The Master has spoken—
http://www.straightdope.com/classics/a4_104.html
There doesn’t seem to be a good asnwer, at this time. 
There is no real cold season. They can occur at any time. The incidence increases slightly in the winter, but generally only in northern climes where the temperature is lower, and people tend to congregate inside rather than out. The difference between summer and winter in terms of cold infections isn’t very significant.
As for the flu, that’s another story. Perhaps our resident expert in epidemiology can answer that, along with corrrecting or refining my above assertions. Jill?
I mean virology and other associated bugs! Not that she doesn’t know more about epidemiology than me!
I’m no Doctor, but QTM is correct about people congregating during colder weather, although it’s not just northern folks who do this. Down here in Texas it’s the same thing, someone in the office comes in sneezing and coughing, spreading those nasty germies everywhere! Someone brings it home and their kids take it to school with them where it spreads like wildfire!! I believe the flu is the same, anytime you have people cooped up closerthanthis, you can spread colds and flu quite effectively.
I do not have any hard scientific data to cite, but retail pharmacies usually have a much busier time in the second half of the year, especially the winter months (cold and flu season). You see a lot more of Avelox being dispensed in January than you do in June, thats for sure.