I'd certainly defer to Sqad (sans u[i ) but, in doing so, I'd just as certainly reinforce the issue of sinuses being a major souce of viral transmission to the rest of the body.
I first made sense of this several years ago when a Doctor friend pointed out that nearly all 'colds' announce their arrival via a sore throat. After a brief schooling on the physical structure of the throat, he pointed out there was absolutely no reason for the sore throat except for the drainage from the sinuses.
Did you ever look in a mirror at the back of your throat just as the telling symptoms of pain begin... notice the two or more red streaks running vertically along the back of the throat?... sinus infection drainage!
Look, sinuses do wonderful things but the dark, damp, mucosa lined passages are, as Sqad intimates, hot beds of infection just awaiting a trigger from a virus or even bacteria. The saving grace is that if one keeps the sinuses draining normally by keeping them well [i]irrigated] a lot of mysery can be avoided.
The real culprit not directly mentioned by the venerable Sqad is the desert like dryness caused by the "out in the cold, back to the central heating..." experienced universally in our cold northern climates, especially here in the U.S. Carrying almost religiously, a small squeeze bottle of saline solution and applying the contents to the sinuses several times during the day, will work wonders.
One wants to accomplish this feat in private since to get maximum effect one has to sniff forcibly inward when spraying and continue to sniff to get the solution as widely spread through the sinuses as possible.
Be assured, there's no medication of any kind in the saline solution (a popular brand here in the U.S. is "Simply Saline" made by Arm & Hammer... it's cheap but one could make their own if so desired.)
So... mind all those things your Mother told you... wash your hands, don't rub your eyes, don't pick your nose (in Sqad's case also be discriminating about whom you kiss!) and... irrigate those sinuses!