Immune To Mental Illness

If a person is a Human and has a Human brain, then they are susceptible to "mental illness", like having a physical body means they are susceptible to illness and injury in general.
The brain is an organ of the body, and it's by far the most complex, and extremely delicate, and completely attached to the rest of the body and blood supply, just like every other organ. Any organ of the human body can become "ill" or be affected by all kinds of factors.
Human beings seem to have no problem accepting that any of their OTHER organs are delicate and susceptible to illness, but when it comes to the main one inside of their skull, most people desperately want to believe that THEIRS is different than everyone else's (and different from all their other organs) and therefore immune to any kind of pollution, injury, illness, or malfunction. 
(If a person's brain is made of silicon or play doh, then they are not susceptible to mental illness.)

Play Doh ROCKS!!!