There's another factor in winter: Warm dry air and low humidity cause
sinuses and nasal passages to dry out. As a result they become more
susceptible to infection.
I know I had a lot more colds up north but some of that is just that
DC is a big petri dish, collecting germs from around the world. You
are around people sneezing and blowing their nose all winter. New York
City is the same way.