Alzheimer's. I watched my father turn into a husk of himself; on the outside he looked the same, but there was nothing left of him internally. I don't think he recognized anyone, you could not have a conversation with him, he reached the point where he could no longer feed himself. And ridiculous though it was, I was so MAD at him for disappearing and leaving me stuck with my mom. Now every time I put something in the wrong place or can't remember a word, I wonder if it is starting for me.
Anything that is mentally degenerative would be bad. I don't believe in the spirit and our bodies are pretty much just meat sacks wrapped around skeletal structures. If my mind is gone what am I really? But that's really more of a dread that a terror. If anything terrifies me, it's got to be any flesh eating disease. Cancer would suck, same with liver failure, heart problems, etc. But I think I could deal, even if I just had to resign myself to my fate. But having my flesh rot off my still living body? Fuck no. I'm out.
As the son of a mortician, having seen so many people after the worst day of their lives: none of them. Haven't met one, yet, I can't beat. I may yet meet that one, but until then I intend to keep forging ahead as I always have and just trust that my genes, and jeans, will carry the day... though, I have to admit that I am getting older and, losing some traction.
But, truthfully, none of them. I've had two bad bouts of flu. Beat them both. I annually was healthy, while my sisters were annually sick. I have ridden with a terrified Medic in a military Jeep sitting next to a guy who might've had Hepatitus. I did what the Medic was supposed to do, I sat calmly next to him, and said "you are going to be fine, don't let these guys scare you, they are just cowards for no good reason." No, I don't know how he faired, I know that I faired fine... and I didn't know how bad Hep is (at the time) but I DID know what the Medics expression meant, and I also knew that the Medic was going absolutely the WRONG thing!
Long and short: I've been exposed to a thing or two, or three, or four, and am the guy who most say 'never gets sick', though I do on occasion... and, I think it's as much mental as otherwise: if you know you can beat it, you can.
I'm not actually terrified by any disease. I do, however, watch a few with interest. C-diff and some others that have become so resistant to antibiotics are fascinating, as are the diseases like Ebola, Marburg, and some of the other zoonotics that jumped over to humans. I'm waiting with what a few friends say is sick fascination for what biologists call the "next big one"-the next pandemic-think Spanish Flu-that kills off a good portion of the human species.