Mental illnesses are characterized by disorders of the psyche, or mind. The view of objective reality is impaired, as are also the patient's self-correctness, his attitudes towards others and his behaviour. Some mental diseases are the result of primary affection of the brain, followed by a disturbance of the body as a whole. Others are caused by diseases of particular organs, with secondary disturbance of mental functions. They may be manifested in a variety of disorders: false sensory impressions, disturbances in thinking or mood, disturbances of consciousness and memory, and intellectual decline. Three types of mental illness are delineated; the psychoses (including schizophrenia and manic-depressive psychosis) constituting the most important group. The second group are nervous and mental disorders, including neuroses, psychopathies, and other nonpsychotic diseases. The third category is mental retardation.
The course of mental disease varies from single or rare attacks with complete remission, to severe, chronic psychoses with gross disorganization of mental activity and deterioration into feeblemindedness.
Significant mental disorder is clinically defined as a significant behavioural or psychological dysfunction that is associated with (a) present distress (a painful symptom) or (b) disability or impairment of functioning or (c) with a significantly increased risk of suffering death, pain, disability or an important loss of freedom. Another definition of mental illness is a non-organic, social-psychological disorder in which the individual is unable to protect his ego or social self sufficiently to participate in ordinary social life and obtain at least a minimal degree of social and psychological rewards.
Since classical times Western thought has entertained three major explanations for behavioural disturbance: demonic possession, moral depravity and illness. Only since the latter half of the 19th century, with the ascendency of scientific medicine and psychiatry, has the illness paradigm achieved prominence. A recent trend, particularly in ethics and the law, to define mental illness as incompetence or irrationality.
In 1984, the World Health Organization (WHO) estimated that at least 40 million people worldwide suffer from severe mental disorders, 20 million suffer from epilepsy, 200 million are incapacitated by less grave mental and neurological conditions, and countless millions are affected by alcohol and drug related problems. A study released that same year in the USA showed that 18.7% (or nearly one in five) of the adult population suffers from at least one mental disorder and a well-known British psychologist has said that a child born in the UK today stands a ten times greater chance of going to a mental hospital than to a university.
In 2001, the World Health Organization reported that mental and neurological disorders – ranging from depression to Alzheimer's and epilepsy – affect 400 million people globally and are set to surge in the next two decades. By 2020, it was predicted that depression would jump to be the second greatest cause of death and disability worldwide, following ischaemic heart disease.