Beloved British author Terry Pratchett is suffering from a rare form of Alzheimer's disease. The writer, 59, shocked fans with the revelation about his illness on Wednesday (12Dec07), after posting the news on the website of artist Paul Kidby, who created the artwork for Pratchett's Discworld series of books. Prachett underwent medical tests earlier this year (07) after having problems with hand-eye co-ordination, and at the time it was suggested he had suffered a mild stroke. But he insists the disease will not prevent him from continuing with his literary career and remains positive about his future. He says, "I would have liked to keep this one quiet for a little while, but because of upcoming conventions and of course the need to keep my publishers informed, it seems to me unfair to withhold the news. "I have been diagnosed with a very rare form of early onset Alzheimer's, which lay behind this year's 'phantom stroke'. We are taking it fairly philosophically down here and possibly with a mild optimism... "Frankly, I would prefer it if people kept things cheerful, because I think there's time for at least a few more books yet." Pratchett has sold more than 55 million books worldwide.