Does anyone out there know the basis for visa refusal on health grounds (apart from obvious AIDS etc)?

Is it true you can be refused for being overweight (and what are their defintions of overweight?)

Also, I need a one off minor operation in the UK - should I delay my application submission until I have had the operation - scheduled for Dec this year. It has no long term health implications, but surgery always carry an associated risk

Thanks in advance for any info!
I am really keen to submit my application (skills independent) but don't want health issues to be the cause of refusal, and will wait if this is sensible.

Caroline