Alzheimer's has got to be the most tragic disease for intellectuals and authors, I would think.
Does anyone remember the movie Iris, based on Iris Murdoch? I saw it once when I was younger, and I can't recall the specifics, only the overwhelming feeling of tragedy that hung over me for days afterwards.
I only recently discovered Pratchett, starting with Soul Music. I thought he was wonderful. And that's one of the great things about literature, I suppose. No matter what happens or how many books he writes, they'll always be there, and his influence will continue long after he's stopped writing.
Terrible news.