Terry Pratchett, the bestselling author of the Discworld fantasy books, is suffering from a rare form of early onset Alzheimer's.
-- "Prachett Announces He Has Alzheimer's," The Guardian, Dec 12, 07
You can read the statement Pratchett issued here. It has his usual touches. I'll be spending the rest of the day thinking of the Mike Resnick story "Winter Solstice."