First of all, I think the distinction between the two is nothing but a crappy heuristic. Physical illnesses manifest themselves mentally, and also the other way around, so what I'm talking about here is not "physical vs. mental", but rather where society has irresponsibly drawn the line between the two in it's faulty perception.
I was thinking about how people in hunter/gatherer societies would have perceived physical illness back in those times. They knew nothing about the human body, and though physical ailments would have commonly been experienced by people back then I wonder if people got who got sick somehow in those days were regarded with the same sort of fear and prejudice that mentally ill people are regarded with today...
Allow me to elaborate.
These days, when someone gets sick, be it cancer, tuberculosis or whatever, we recognize that something hashappened to them. We don't take it to be an extension of their person and we don't judge them for it.
Mental Illnesses I think are regarded differently. When most people talk of mentally ill people, they talk as if it is the person him/herself that is faulty, in contrast to with physical ailments, where something has happened to the person.
This at first seemed very strange to me... my original explanation was that mental processes are simply not as well understood as physical processes and because of this, people "mystify" mental illness to some extent, taking them to be some sort of cosmic personal failure springing from the core of the person (think dualism) rather than pitfalls of circumstance.
Okay, that sort of works... but it doesn't dig deep enough.
What I think is really happening is this:
In hunter-gatherer societies, the primary arena of the human struggle was the survival arena, or the physical arena. Their struggle for survival and procreation was probably the only insight most of them had into themselves. It was their primary reality; it was what gave meaning to their lives. So therefore, when someone lost that physical battle in a non-obvious way* it was likely seen by outsiders as a failure of being.
After the agricultural revolution though, things began to change. The struggle of the survival arena was still paramount, but humanity was winning the battle at this point... and holding a lead is always easier than playing the comeback team (sports metaphors ftw)...so what did humanity do with the remainder of it's energy? Slowly, fractions of the human consciousness drifted away from the physical struggle and bit by bit humanity began to enter a new arena... the ego arena.
What is the ego arena? Basically what we are in today. The struggle to affirm ones ego is the struggle of modern existence, and let's be clear here; I'm not talking about selfish behaviour when I'm talking about the affirmation of the ego, I'm talking about the drive to be considered valuable or worthy by other people. This is the arena we are in now. Our struggle as people consists not in finding our next meal or avoiding the next predator, but in reaffirming our sense of worth by somehow improving (or perceiving to imrove; people do this in many ways including self deception) the lives of others.
Back to mental illness.
The reason mental illness is viewed differently in society today than physical illness is because it represents either a failure to partake in, or a loss in the primary game of human existence, the ego struggle. When people lose this struggle most people view them as defective people because right now the ego struggle is what gives meaning to our lives, and subsequently defines us as humans. The physical struggle is now viewed by the common man in a more abstract way; the physical struggle is all but won. We have all but transcended it and so subsequently it no longer defines us as people. This is why, in contrast to mental illness, people don't view physical ailments as something wrong with the person.
So what's next after the ego arena? The experience arena of course! Once society begins to transcend the struggle to affirm their self worth (this is already happening slowly), the new struggle will be to experience profound and amazing things as much as possible.
What will be the major shift in history for the victory over the ego struggle? As the agricultural revolution was the turning point in the battle that will eventually render the physical struggle obsolete, some sort of transhumanist revolution will be the turning point in the battle to transcend the ego arena and therefore the woes of the modern human condition.
How to begin this revolution? Legalize all recreational drugs, and slowly work to remove the stigma that surrounds them. That will be the first step.
*when I say in a non-obvious way I mean in a way that does not appear to be arbitrary. For example, even in hunter-gatherer societies, if a person was mauled by a bear it would not have been viewed as a fault to the person, because it is very clearly beyond that persons control. A person who was born physically weak however, or gets some sort of chronic condition that makes them unable to fight in the physical arena would have been viewed as faulty. Today, obvious mental illness is recognized, but people with the precursors to this illness sare viewed as faulty. See the connection?