As far as i know, the chance to get a flu is not dependend strictly on the absolute temperature value, but on the temperature gaps you experience between room/outside and such. (Or AC on/off).
Also, when it is cold in the winter people are dressed appropriately for ti, while in the summer when you enter a 18 degree air conditioned office with your short shirt, it will be colder for you than in winter time.

Just to note to my response above, there is some connection to temperature, since the body body does need to put some resources on warming up when in cold environment and not dressed properly, and then the immune system tends to lower its activity.

Thanks, interesting.
I am not sure the human and guinea pigs immune systems are so similar.
And maybe i also refered to general kinds of 'cold' deseases and the flu has a specific more temperature oriented specific mechanism.
But i am far from being an expert in that stuff, hopefully someone with more knowledge here will share some of it with us.

(http://www.haaretz.co.il/hasite/spages/1105167.html)