I spoke to the consultant chest specialist that is dealing with Billy, who now has been diagnosed with pigeon lung and there seems to be different types or stages.
He has had test results back confirming pigeon lung, but they are awaiting other test results to see if he has the worst form of pigeon lung, which is a fungus in the lungs which makes him totally alergic to pigeons and any dealings with them could kill him.
As I say not sure if it is different types or stages, I will ask the consultant when I meet him next :-/