Coughs and colds

What is it, especially during the colder months?  One cold after another.  I work in a large building with many other people and, no opening windows and, it seems to be one cold after another for many of us at the moment.  Any tips?  On top of that I make a lousy ill person, I mope!  As a result I haven't been to the gym, I haven't given up but hate to have unexpected disruption to the routine.