Many factors may contribute to the disturbed functioning. Heredity may be a factor in mental illness as it is in diabetes and cancer. Stress may contribute to the onset of mental illness in a vulnerable person. Recreational drugs may also contribute to onset, but are unlikely to be the single cause. Since the cause of long term mental illnesses are not known, there is no effective prevention at this time.
More research is needed to determine causes and to plan strategies of prevention.
Likewise, there are no cures for mental illnesses. However, there are treatments available which can substantially improve the functioning of persons with these disorders.