I don't think the cold weather will make you sick by itself but it might make it harder to fight off a virus or bacteria. I have ridden a couple of miles in 25 degree temps wearing jeans and a tee shirt and it has never made me sick. I have started coughing after breathing in a lot of cold air during a short sprint but it only lasted a couple of minutes. I wish I had a dollar for every time somebody told me I was going to catch a cold.