Factors separating 'mental illness' from 'eccentric'

Mental illness is a serious event in the life of a person and those who are involved with the individual. Briefly discuss some of the factors that the separate 'mental illness' from 'eccentric' or odd behavior and thought processes (we covered these in class and they are noted in the readings from your textbook as well.) Provide adequate detail and examples in order to answer this question for full credit.