Terry Pratchett just announced that he's been diagnosed with a rare form of early-onset Alzheimer's. He's at least somewhat upbeat, though:
PS I would just like to draw attention to everyone reading the above that this should be interpreted as 'I am not dead'. I will, of course, be dead at some future point, as will everybody else. For me, this maybe further off than you think - it's too soon to tell. I know it's a very human thing to say "Is there anything I can do", but in this case I would only entertain offers from very high-end experts in brain chemistry.
I'll resist that "very human thing" with difficulty. But best wishes to him and his family as they go through what must be a very difficult time.