What sort of mental illness have you witnessed?
The affliction of shuffling around the playing hall with a plastic carrier bag in hand (Common in the UK). The wearing of obnoxious sweaters (Worldwide), Saying the word 'Check', when it is pretty obvious that indeed you are. The list is endless

As a club and occasional congress player, I am saddened by the high percentage of chess players obviously suffering from ill health. I notice rather a lot of obesity and noticable mental illness just via conversation and body language. I feel that each countries national federation should have an healthy lifestyle director or similar to give advice and so on.
I welcome everyones comments.