Author Terry Pratchett is suffering from a rare form of early Alzheimer's disease, it has been revealed.
The writer said he would continue to work on his latest book
In a letter published on the website of artist Paul Kidby, the writer said the condition was behind a "phantom stroke" he had earlier this year.
Pratchett said his statement should be interpreted as "I am not dead" and that he had taken the news "philosophically" and "possibly with a mild optimism".
The Discworld author, 59, has sold more than 55 million books worldwide.
"I would have liked to keep this one quiet for a little while," he wrote in a letter headed 'An Embuggerance'.
"But because of upcoming conventions and of course the need to keep my publishers informed, it seems to me unfair to withhold the news."
The author said work was continuing on his latest works, Nation and Unseen Academicals, and that there was "time for at least a few more books yet".
"All other things being equal, I expect to meet most current and, as far as possible, future commitments."
"Frankly, I would prefer it if people kept things cheerful," he continued, saying it was "too soon to tell" if the condition was immediately life-threatening.
"I will, of course, be dead at some future point, as will everybody else. For me, this maybe further off than you think.
"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."
Pratchett, whose work has been translated into 27 languages, was made an OBE in 1998 for services to British literature.
The Discworld series is set in a parallel universe supported on the backs of four elephants that stand on the shell of a turtle.
Making Money, the 36th novel in the sequence, was published earlier this year.