Mental illness is a physical disease, not something to do with "disillusionment" or anything like that.
Depressed people can be disillusioned, of course, but that is a symptom, not a cause. The cause is still somewhat mysterious, but it's biological, physical, not an attitude thing.
According to what? You assert that the cause is "still somewhat mysterious" (you mean completely?) in the same sentence you claim the cause is biological. That's pretty contradictory.
No it's not. You can have an idea of the general area where the cause lies without knowing precisely what it is.
For example, there are clear genetic factors in mental illness, which can be seen in the same way that genetic factors for any disease are studies. This doesn't tell you which genes cause it, let alone how they cause it, but it still tells you that it's a biological thing.
There are not. There are no conclusive genes or genetic markers for depression. All that is required for someone to be depressed is to self-report as depressed and experiencing those symptoms.
