Damn, damn, damn.
I hadn't heard of this before, as I don't follow Terry Pratchett's work much. But I wouldn't wish this on anyone, let alone a writer. The whole concept of losing your memory is so scary.
Two of my uncles died of Alzheimers, so that makes me nervous. My mother is in her 80s, and she's fine, though, and so are her sisters. In my uncles' cases, I think there may have been other factors involved besides heredity.
Anyway, let's keep our fingers crossed for Terry Pratchett. And they are working on treatments for the disease.