"Those who run or walk briskly for 35 to 45 minutes per day, five days a week, spend about half the number of days per year suffering from cold symptoms, compared to sedentary people. In addition, about 60 per cent of recreational runners experience fewer colds after they initiate a running programme, compared to when they were sedentary . . . .
And mum probably forgot to tell you that if you run about 100 or more km per week, your risk of illness is about double, compared to the individuals who log less than 32 weekly kms. That suggests, of course, that immune function declines as training volume increases, yet we also know that runners tend to be healthier than sedentary people. So, a very reasonable question would be: what number of kms per week is best for overall health? Even mum herself doesn't know the answer to that question. The epidemiological evidence indicates that running a moderate amount of km is better for your health than staying on the couch. However, running lots and lots of km is worse for your health than being moderate: mega-mileage is associated with lots of sniffles, coughs, and perhaps other problems. Basically, the graphical plot of illness frequency versus mileage is a U-shaped curve. If there's no mileage, there is quite a bit of sickness, middle mileage leads to little illness, and high km let the microorganisms rule again. The 'middle ground' of best health may well be 25 to 50 km per week or so, or 35 to 45 minutes per day, but no one knows for certain, and the optimal amount no doubt varies from person to person."
The way the article (and that last sentence) was written made me think of my Dad and his theory on dirt. He's said many times that dirt is actually good for you, we just don't know how much or what kind!
So, we can probably all agree that exercise is good. But as in most things, too much of a good thing is not a good thing!