I think there are just some people that are more prone to get sick from respiratory viruses than others. I have a friend and every time I talk to him either him, his wife or his kid are sick. On the other hand I've been sick 3 times since I was ~12. I don't remember being a sick kid before then, though, just don't have vivid memories of being sick, like I do after. I got the flu when I was in 6th grade because I remember I had to skip our family Thanksgiving. Then the summer of my junior year I got something that resembled mono, it pissed me off because I had to test out my lifts for varsity football during that time and I had no energy or strength. Then about 10 years ago we traveled down to Florida for a friends wedding and I got basically no sleep with the wedding and the driving through the night, etc. that when I got back I had a pretty bad 24 hour bug. Other than that outside of occasional sniffles, which don't hinder my ability to do anything I've gotten nothing. So, 3 times in about 27 years. I also never got the flu shot, I figured if it ain't broke don't fix it, but I actually did get it this year, just to be a good citizen. My best guess is it's a combination of exercise, protein intake, and sleep, but that's literally just a guess.