This is copied from a now closed thread (Thanks Shaft109) but it belongs here:
I have a definitive answer for you.
Recently went to a combined AFCO (all services) with regard to finding out one thing from source itself before I was too late. Any prizes for guessing what that was?
Me, entered the said building and went to the RAF Sgt on the desk.
Sgt "What can we do for you today?"
Me "I'm interested in joining but I have a history of Asthma"
Sgt "It's now law that the Armed forces cannot accept applicants with any history of asthma" (Note ALL Armed Forces, any part thereof including Airman trades).
The short story is they don't want the hassle.
For hassle read 'our medical services are so overstretched and under manned that it is simply counter-productive if we recruit people with actual or potential medical problems. Furthermore these problems may limit your future employment (Service speak for OOA or arduous jobs etc).