Personally, my experience with volume and health goes against what one would expect. I seem to get sick more frequently at low volumes than I do at high volumes.
That said, it is said that running higher volume places a higher toll on your immune system, which makes it harder to fight off sickness. I don't know if my experiences go against this because I'm focusing more on taking care of my body when I'm running more or what.
I usually don't plug this type of thing but Lisa started buying this stuff called Airborne after getting sick nearly every other week, passing colds off to me twice in the month before Chicago and two or three times shortly after Chicago. I don't know if it's part psychological or what but she has been much healthier and I haven't had a cold since she (and occasionally I) started taking that stuff. You can find it in any place you would find cold medication.