IMO (not necessarily scientific), if you dress properly, cold won't affect your resistance; otherwise you need to use energy to overcome the cold and could be more susceptible; the same could occur if you're too hot in the summer. My wife and I ride early AM and are alone on the trails, but encounter lots of individuals on the return whether it's an out and back or some kind of loop. Nobody is coughing or sneezing now, and since the trails are pretty wide, it seems safe; however if there are individuals coughing and/or sneezing later in the year, it could be a problem.
Possibly I'll mask up for the return.