It might be interesting to consider that it's not necessarily the race that lowers the immune response in a competitor.

Most of the serious racers are well into double digit hours on a weekly basis, with multiple efforts above various threshold levels. Heck, if there is one common boast on here it's just that. The race will be run at lower levels of effort, but the training in advance may well have lowered the immune response in the individual to the point where in a large crowd of untested individuals racing or supporting, they have become more susceptible than a weekend warrior..

I offer not a single fact as evidence, and no professional chops whatsoever, other than the somewhat accepted fact that at peak fitness, we are not necessarily at peak immunity.