[Closed] why am i getting ill so much!?

Relative to my non cycling friends, i eat more heathily, get more sleep, drink less, work fewer horus and do more exercise. Yet i seem to be ill more often, about once every 1-2 months. Had the flu recently, barely got over it then got a cold. The after affects are the real pain, as it really seems to affect my fitness for a week or two after i get better. whats going on??
is it cycling? i commute by bike every day (20 minutes each way), could the cold weather be doing it?