“It is okay to exercise if you have a simple head cold or congestion — in fact, it may improve the way you feel. I would avoid heavy, prolonged exercise with a head cold, though,” since it can unbalance that important T1 and T2-helper cell response.

And take comfort in the results of the most recent study to look at actual, practicing marathoners. In it, 1,694 runners at the 2000 Stockholm Marathon informed researchers about any colds or other infectious illness they developed in the three weeks before or three weeks after the race. Nearly one-fifth of the runners fell ill during that time period. That’s higher than the rates in people generally, but it still means that the overwhelming majority of runners didn’t get sick.