I'm not convinced there's a significant increase in disease immunity from exercise. I sometimes go years in between colds or fevers. And sometimes I get 4 or 5 bugs in a 10 month period. I spent 12 years running 6 dys/wk (gotta have a recovery day or you gain nothing), and have had several extended periods of no running, return to running, no running, return to biking, etc.. During those periods I didn't notice any correlation between the times of regular exercise and times of more or less frequent colds or flu. I HAVE noticed a pompous superiority complex in some runners who claim they never get sick ...and insist everybody should do just as they do.

I will forever be committed to regular exercise. But when the kid brings home a cold or flu strain you haven't had, you're going down. But, being stronger from regular exercise, you are more likely to survive, or maybe barely notice it.