Its is a respiratory disease im affraid......none curable but very preventable and to some extent managable. Its usually found in badly ventilated barns where dusty conditions agrivate the horses wind pipe and lungs, and cause an alergic reaction (similar to asthma I suppose?) Bad mouldy/dusty hay is also a culprit although the exact cause isnt proven clinically.
You may have heard of its technical term COPD?? That is more regularly recognised as heaves is an old fashioned term that many don't recognise.
Hope that helps,