First time my youngest did a cross-country skiing race, his coach heard him wheezing as he went by a check point and nabbed him out of the race. Exertion asthma. For a while there he used an inhaler, but after a few years his lung capacity had expanded to where we've never heard him wheeze any more. And yes, the doctor confirmed, exercize can cure some kinds of asthma. But then _more_ exercize can bring it back. Some of the really elite x-country skiers have it bad, not so much from exertion as from pulling serious masses of seriously cold air into their lungs.