I developed asthma last year - convenient timing when I wanted to start a career in aviation.
I did the honest things and declared to the CAA that I was using a daily inhaler and they asked me to do the excercise test - which I failed. This involves you running around the building at Gatwick for 5 minutes or whatever until you are out of breath. They then repeat the peak flow.
After consulting with my doctor we increased my medication and the next time I did the test I passed so it is possible to pass it.
My theory is as follows - this is all my own personal thinking and not backed my medical opinion - so make of it what you will !!
Asthma is a constriction of the airways. This can be caused by many things. One of the things that makes an episode worse for sufferes is that when the symptoms start they start breathing faster - this irritates the airways more and hence makes the episode worse.
I could be totally wrong but I think that this is why the CAA make you run around. Now, I think if you are fit and excercise regulary then your body needs less air for a given amount of excercise. Therefore you dont irritate your airways as much running around Aviation House!
Getting back on subject, you can have asthma and be a pilot and have *certain* types of inhaler providing you can pass the excercise test if you are given one.
My advice is to be prepared.
This is going to become more common in future years as more and more people are developing forms of asthma.