I was thinking this too. I wouldnt be surprised if it had something to do with all the traumatic events in his life. I'm sure the time he spent getting tested on in that lab didn't do him any good. He was also with a few people that didn't know how to take care of a chimp. Then there's the booze and smoking.
I guess his brother died at about the same age of the same ailment, so maybe it was hereditary also. ============================== He lifts me clear to the sky, you know he taught me to fly.