[mcguire]

"Many people have mental health concerns from time to time. But a mental health concern becomes a mental illness when ongoing signs and symptoms cause frequent stress and affect your ability to function."

http://www.mayoclinic.org/diseases-conditions/mental-illness...

"A mental disorder, also called a mental illness or psychiatric disorder, is a [...] behavioral or mental pattern that may cause suffering or a poor ability to function in life. Such features may be persistent, relapsing and remitting, or occur as a single episode. Many disorders have been described, with signs and symptoms that vary widely between specific disorders."

https://en.wikipedia.org/wiki/Mental_disorder