I do find that I seem to get whatever "bug" is going around more easily than others, however, I'm not 100% sure if it's due to my autoimmune diseases (lupus, fibro, sjogren's, chronic fatigue) or if it's at least in part due to medication. I've been on prednisone quite often in my life due to my asthma and the auto immune diseases, and definitely think it has pretty much "ruined" my immune system - I know for a fact, that when I am on it, my resistance is much lower (which it can do to anyone who takes it), but I also feel like even when I'm not on it, my resistance is lower than other people's and even lower than what mine used to be..
I do agree with TrudieC also that it happens to me a lot more often when I've overdone it physically or haven't gotten enough sleep (which, unfortunately, is most of the time!)
I am in the same boat and agree that the fighting off is the worse. I do end up getting more colds and things but I think this happens more often when I have overdone it physically, stresswise or if I am not getting enough sleep.
I have been diagnosed with those three ...plus. What I notice with myself is that I experience a long period of feeling like I'm fighting something off. I often do not come down with what I've been exposed to or if I do, I feel better once I am symptomatic. The fighting it off is the worst part.