Mental illnesses are largely medical illnesses where sometimes stress could either trigger or contribute to the onset or maintenance of it .

Yes stress is part of everybody's experience . Most people realize that they do not have any stress which is responsible for the change in their mental state. They have to have a biological vulnerability , for stress to cause illness.

No. People with certain personality styles/ personality disorders, poor coping skills are vulnerable . Others with traumatic early childhood experiences , loss of a parent for example , solitary lifestyle etc. are vulnerable .

Earlier when the role of the brain was not understood , any abnormal behavior was attributed to the spirits, magical influences etc. With the knowledge of the way the brain functions and the availability of medical management of mental illness there is no doubt that these are brain disorders .