Depression is born of anxiety and anxiety is a human condition that is born of fear - a fear of failure, a fear of inadequacy, a fear of persecution and a subsequent fear of the future. In time, the sufferer will tip over into depression and then even the smallest worry becomes insurmountable, and they find themselves unable to go outdoors or function normally.
It is impossible to quantify and equally impossible to generalise about individual cases and so treatment and an eventualcure is an illusive goal that is seldom achieved.
Be strong for those that cannot cope. Carry them with you on their journey.