I think is very important to distinguish between mental diseases and mental disorders. A disease is when there's some biological or chemical alteration at the level of the brain that produces this physical symptoms. However, the mental disorders are an expression of a problem at the cognitive level but not at the biological level. For example, if you know the anxiety disorders like Obsessive Compulsive Disorder, or the social phobia, or the Disorder of Generalized Anxiety, these are mental disorders, but not diseases.
It means, than pills can't cure these kind of disorders, and the cure has to arrive at the level of working with a psychologist, psychoanalyst or some kind of alternative medicine. I had for many years Obsessive Compulsive Disorder, and the basics for curing the OCD is to be able to learn to not to believe your thoughts. It requires practice and patience, but it's something that can be learnt.