My first semester away at college I was so sick for so long and I couldn't figure out why. It was sort of a cold/congestion thing, never enough to keep me home but always enough to keep me feeling crappy.
It started in my sinuses, then moved to my ears (I couldn't hear properly out of my left ear for over a week, that drove me nuts) then moved to my nose, and throat and finally settled in my chest. I seriously was sick from about the second week of school well into midterms.
Finally, when I stopped smoking up, I got better.
I watched the rest of the kids on my floor spread one sickness after another back and forth between them, but I always refused to smoke with them. My roommate wound up with pnuemonia 2nd term, but I was fine.
Ever since then I've always been wary of toking and my immune system. If I'm already tired or not feeling 100% I'll usually pass, or if the other person has a cold or looks sick I'll pass too. The way I look at it is that I've got the rest of my life to get high, why get sick?