I suffered with asthma when I was 8-14 and swimming was the best form of excercise I could do to get proper exercise, as I've grown up now my asthma isn't much of an issue when I train, although I am one of the lucky ones who have seemed to somewhat grow out of the asthma, I still occassionally get symptoms, but I only treat my asthma with a low dosage, and a reliever, as long as I have my reliever handy when I train boxing, I seem to keep up like everyone else. I have my peak flow measured in an asthma clinic twice a year and its always well above average for my height and weight, so it seems that swimming with asthma has attributed to me having quite a good breathing capacity.