As I said, it's a gamble. Every company does it different and most are shit at recruiting. I'd play it safe myself and leave it out and if they happen to ask about it later I'd be honest.
I don't see a reason to mention the sickness unless they ask for it OR unless you has to quit your previous job for said sickness. It's kinda weird if you can't mention why you stopped your last job. :P
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I've said this before but I'm quoting it again...don't ever put anything negative in a cover later, it will lead your CV straight to the trashbin.