My mother died from Alzheimer's disease, as did several of her siblings. Mom was one of 17 children. Her father had Alzheimer's and passed that gene on to more than half his children.
As an only child, I had the job of watching my mother slowly be taken away from me by this terrible disease. She kept asking me if she had the disease, and I would lie to her and say "No." Lying was easier than watching her become upset and obsessed with the fact that she had Alzheimer's.