So mad people live without illnesses, and probably die without they (the illness) being the cause. Not that they do not have illness in their body but that those illnesses refuse to work or have effect on them whilst they are alive.
We cannot say for certain that this is what killed them as there are and may be several illnesses in their body at the time of their death.
What we know is that mad people have a shorter lifespan and that’s what we should be concerned about. How do we make them live longer without they committing or thinking about suicide?
The answer is by caring for them, and loving them. Their madness may never leave (we have a saying that when you treat a mad man, a bit of it still remains in him) but what can leave is… I don’t know.
P.S.: Cause of death of mad people, I deem it to be more spiritual than biological.