My biggest culprit is sinus infections. I've been fighting with my ENT for years trying to get a FESS done, because I have a deviated septum (even I can see it on the Xrays), and I know that has a lot to do with why I can't get rid of them.
So at least twice a year, spring and fall allergy seasons, I'll wind up on antibiotics. I start eating yogure as soon as I start taking them and thast usually helps.
But this past winter, when my oldest wound up in the hospital, because of all the stress and lack of sleep - my sinus infection spread to bronchitis, spread to pneumonia - and I was on antibiotics for almost 3 months (different ones - I think there were 4 different kinds). Plus steroids and breathing treatments. And Diflucan became part of my "diet", too.
On the other side of the board - my 16 yr old son has only had antibiotics 5 times in his entire life. 1 of those times was for preventative when he had dental surgery, and another was when his appendix ruptured. He has had 1 ear infection, 1 sinus infection, and 1 case of bronchitis. That's it! Of course he has only had the flu twice and has had the chicken pox, He just doesn't get sick. I've often wondered if the CDC would take him away and study his immune system.