The concept of mental health – and even more so its converse of mental illness – has become ubiquitous in the modern West, and it deserves serious examination by philosophers. Many, probably most, cultures would not recognize the claim that a mind that sees demons or refuses to speak or commits suicide is in a condition analogous to a body with a fever or a broken limb.
The idea of mental health and illness is the central idea in the psychological approach that we typically refer to as the medical model. The term “medical model”, in its most basic sense, means that one approaches a given field of human endeavour in the manner associated with medicine: that field may then be considered a part of medicine, or simply analogous to it. I believe the term was coined by R.D. Laing, the prominent critic of psychiatry, and so it often takes on a negative cast, for the application of specific aspects of modern medicine in areas where it is inappropriate to do so.
It does not have to, though. Unless we reject modern medicine in its entirety (which would be a stupid idea), we are going to accept some aspects of the medical model for at least the practice of medicine itself. Modern medicine has accomplished a great deal, even in its application to phenomena of the mind: antipsychotics and antidepressants are not cure-alls by any means, but for a great many people, their mental lives are much improved as a result of these medicines.