My grandfather died from Alzheimer's and it was really awful. He got to the point where he couldn't even speak anymore. But for me, the hardest thing I think I ever witnessed was my Aunt who passed from Lou Gehrig's or ALS. Being totally locked in her body while her mind was fully aware of what was happening. When I think about it too long, I get really upset. That disease terrifies me.
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.
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.