Why am I not as sick now?

So I am pretty much over my latest cold now. I fought the cold for a week and a half before I submitted to its evil. I was still running outside at that point and breathing in the fresh air was definitely keeping the cold at bay. That plus some Zicam. But then the dreaded hip flexor which I have been battling all summer reared its ugly head and I realized that I had to cool it with the running for awhile. Returned to physical therapy 2 weeks ago.
It's amazing what happens when you stop breathing in fresh air, stop physical activity because of an injury. Within 4 days, I had a full-blown sore throat and congestion. I hate sore throats, they are the worst. But I guess a cold once a year is better than 4 times a year.
Rewind to when I was much heavier and smoking. I must have had a cold every season, even summer. Summer colds are the worst, especially with the humidity levels. You feel like you can't breathe. Looking back on it, it's not surprising why I was sick so much. First, I was polluting my lungs and second, my level of physical activity was zero. Also, the colds I would get back then would be awful and would literally knock me out for a week.
Now, many, many pounds lighter and no longer smoking, I don't get colds as often, only about once a year. Well, it doesn't take a rocket scientist to figure out why I am not getting as many colds. The clear lungs from quitting smoking has a lot to do with it. But, does the lighter body weight have anything to do with it? Not sure, but the increased physical activity has made a difference. Many studies have been shown that exercise can boost immune function. This theory probably is true since when I was heavier I would be out of the game for a week, where now, it may only be a couple of days. In fact, I took a sick day to get some rest and the next day went to exercise class because I was going to "sweat out the germs". The other thing I notice with running is when my sinuses are stuffed up, the fresh, crisp air and oxygen was a natural sinus rinse. And let's not forget the benefits of real food either. As opposed to food that looks beige (to use a term from Bob Harper, referring to fried food)
I hate being sick. Isn't that enough of a reason to get healthy?