the common cold you are describing sould like a common viral infection. again, not due to the tempurature. go get sterilized (ie remove bacterial/viral contamination, not the other kind of sterilized), then stand by yourself out in the woods in the middle of winter. you may die of exposure, but you won't get a cold. there no vector for the introduction of desease.
Such studies were conducted in the 20's-50's. people standing in a cold room barefoot, in 1" of standing water. by themselves, they wouldn't get a cold. when in a similar situation with other people, they'd tend to get allot more colds.
One other reason? School is in. The transmission of germs between children in school (and subsequently their families) is one of the major methods for the passing of colds/flu in many western cultures.
One more reason? The cold. I know I said above that the tempurature doesn't have anything to do with getting a cold, but that not entirely true. the Body has to put more reasources into keeping the core tempurature up high enough so that you don't get hypothermia while walking to your car. As such, there is less energy available for keeping the immune system up to snuff. I have not seen many studies along these lines, but IME, the human immune sytem is slightly supressed during the winter months. The lower levels of sunlight may also play a role in the weakening of the immune system.