It seems like people get sick during the coldest time of the year. Mid December to early Feb. If I get sick it always seems to be between mid December and late January. Cold weather definitely seems to effect circulation/blood pressure if your out in it long enough. I notice that because majority of my job is outdoors.
I believe that part of the issue with the common cold is that there are so many different varieties of the rhinovirus, also, the rhinovirus mutates regularly. I believe, that statistically speaking, part of the reason we get more colds in the winter is because we spend more time indoors around other humans. 🙂