Blah, I was about to post that I agree with you AC, and went off in search of a definition to back it up.
But all the definitions seemed to agree with this sentiment.
menŠtal ill|ness.
NOUN
1.a condition which causes serious disorder in a person's behaviour or thinking.
Mental Illness: Learn the Definition, Tests and Statistics
I really detest the term ' mental illness'. I would prefer to use 'psychological disorder, if anything.