My mum is a doctor, i was actually having this conversation with her last week. It's actually quite interesting how it all works. Scientists/Doctors are still not 100% on why it does flare up so much in winter but it does have a lot to do with bacteria prefering cold, dry weather. On top of that there are a heap of other theories. 1 for example is that people are closer together when its cold (sit on the couch closer or whatever) that helps spread flu/cold bacteria.
This is my third winter on the bike and i get no sicker than i did driving the car. The most important thing you can get is a neck warmer/Balaclava to keep your neck warm. When i ride in Winter i usually have my bike boots, Work pants with my Bullet jeans ontop of them, work shirt, Jumper and then bike jacket ontop, with a neck warmer and then gloves. And only my hands get cold (left hand mainly? No idea why). I usually sit my left hand on the engine haha. In saying that i HATE wearing winter gloves. I ride with summer gloves all year round.