which characters in the novel actually have mental illnesses? which ones dont? whats the defenetion of a mental illness anyway?

Everyone but the doctor is mentally ill....... he's just a drug addict. Many of the patients are curable, a few of them don't want to be cured, or maybe they simply don't need to be (McMurphy, Harding, Cheswick, Billy). There's a fine line here.

Definition of Mental Illness: Any of various conditions characterized by impairment of an individual's normal cognitive, emotional, or behavioral functioning, and caused by social, psychological, biochemical, genetic, or other factors, such as infection or head trauma. Also called emotional illness, mental disease, mental disorder.

http://www.thefreedictionary.com/mental+illness