My under-researched model of how this happens, is that there is a lot of tear-and-wear in the muscles where the effort is happening, which leads to inflammation, and that's where the white-blood cells might end up.
"Their first conclusion was that athletes are lousy at identifying whether and why they are sniffling. The original 1980s studies had relied on runners’ self-reports of illness. But newer experiments that actually tested saliva showed that less than a third of marathon runners who thought they had caught a cold actually had. Statistically, their odds of becoming sick were about the same as for anyone else in the race’s host city.
The athletes probably had misinterpreted allergies or short-term scratchiness in their airways after the race as a cold, says John Campbell, a professor at the University of Bath who was a co-author of the new review."
So that looks like the fault of the ends (read: the event) was actually led by the means (training).
I have an optometrist friend who says he can tell easily in eye exam which old people have worn glasses or sunglasses and which haven't.