Cushings Disease is an illness that befalls both humans and animals alike. I believe i read somewhere that JFK had it, and i remember thinking "... but he wasn't covered in long shaggy hair!"
I dont know much about human cushings, or the similarities to horse cushings.. but Horse Cushings is caused by a benign tumor in the pituitary gland of the horse, typically the horse will develop this illness in the later years of life, but there have been odd cases where horses as young as 5 has gotten it. It will cause the horse to lose and have trouble making back muscles.. the horse will get a pot belly and a long shaggy coat, and will be prone to laminitis as well as a weak immune system.
There is no cure to Cushings Disease, but if you diagnose it quick, there are medications they can be on so they can still be in normal health for the rest of their life. Discontinuing the medication will result in the cushings symptoms appearing.
Im not sure a horse can die from Cushings.. i think they die from things like founder or a common cold/staff infection that was made more probable by the cushings. Just like with humans and AIDS/HIV. AIDS/HIV does not kill a single person... it just weakens the immune system so badly that a common cold or mild infection kills them.
I hope that sheds some light on it for ya!