This has been talked about a few times (for good reason - it's an interesting topic / question!). This post is a good place to start - there's a link to a full medical report as well as a couple of breakdowns of the causes of his problems.

In short: he couldn't speak properly, couldn't chew, was impotent & sterile, and had chronic diarrhoea and vomiting along with multiple mental disabilities (paranoia, hallucinations, nervous breakdowns).