American Psychiatric Association defines the Mental Illness in this fashion: "Mental illnesses are health conditions involving changes in emotion, thinking or behavior (or a combination of these). Mental illnesses are associated with distress and/or problems functioning in social, work or family activities." Almost one in five U.S. adults experience some form of mental illness. The good thing is that Mental illness is treatable, and most individuals with mental health condition continue to perform their daily tasks and activities.
The common mental disorders could be Alzheimer's, anxiety disorders, eating disorders, gambling disorders, OCD, personality disorder. View the complete list here.