Writing in 1986, 24 years after his seven-year-old daughter died, he recounted the hours before she passed away.
"Olivia, my eldest daughter, caught measles when she was seven years old. As the illness took its usual course I can remember reading to her often in bed and not feeling particularly alarmed about it," he wrote.
"Then one morning, when she was well on the road to recovery, I was sitting on her bed showing her how to fashion little animals out of coloured pipe-cleaners, and when it came to her turn to make one herself, I noticed that her fingers and her mind were not working together and she couldn’t do anything. 'Are you feeling all right?' I asked her. 'I feel all sleepy,' she said. In an hour, she was unconscious. In twelve hours she was dead."
In pictures: Roald Dahl's most enduring characters
In pictures: Roald Dahl's most enduring characters
1/12 Willy Wonka from 'Charlie and the Chocolate Factory'
Willy Wonka (pictured being played by Gene Wilder in 1967) is a bizarre and slightly unsettling man but who can say no to that delicious chocolate?
2/12 The Oompa Loompas from 'Charlie and the Chocolate Factory'
Despite the characters' questionable evolution from being African pygmies in early editions of the novel, the Oompa Loompas were so fantastical they captured the imagination of generations. The first film's version of the characters are still a popular fancy dress theme.
3/12 Veruca Salt from 'Charlie and the Chocolate Factory'
The girl who "wants it now" is so incredibly vile she ended up in the rubbish after being judged a bad nut by Willy Wonka's squirrels. An American alternative rock band even took her name.
4/12 The Twits from 'The Twits'
Mr and Mrs Twit are horrible, hideous and hateful, coating trees with glue to catch birds to at. But the Roly-Poly Bird and Muggle-Wumps teach them a lesson in the end.
Quentin Blake/House of Illustration
5/12 James from 'James and the Giant Peach'
James is beaten and starved by his cruel aunts after his parents are eaten by an escaped rhino but eventually gets to go on an adventure with the friends he has always waited for on his magic peach, ending up on the top of the Empire State Building.
6/12 Matilda from 'Matilda'
Every little girl who read 'Matilda' wondered what it would be like to be the main character. Apart from th abusive family, the telekinesis must be endless fun.
7/12 Miss Trunchbull from 'Matilda'
Miss Trunchbull, played by Pam Ferris in the 1996 film, is the aunt and headmistress of every child's worst nightmares. Just think of poor Bruce Bogtrotter and that cake.
8/12 The Big Friendly Giant from 'The BFG'
A 24-foot-tall BFG works giving out good dreams to children and saves them all from people-eating giants. Shame about the snozzcumbers.
9/12 The Grand High Witch from 'The Witches'
The Grand High Witch, seen here in the film, must be one of Dahl's most terrifying creations. Turning children into slugs and squishing them, she wanted to destroy them all, she had no hair or toes and claws for hands.
10/12 Mr Fox from 'Fantastic Mr Fox'
Mr Fox outsmarts those silly farmers, feeding his family by killing their chickens and avoiding starvation.
11/12 George from 'George's Marvellous Medicine'
Some people would say that swapping your gran's medicine with poison isn't ok but that isn't the point here.
12/12 Grandma from 'George's Marvellous Medicine'
Grandma was quite a character until she vanished: 'She was selfish grumpy old woman. She had pale brown teeth and a small pucker-up mouth like a dog's bottom.'
Dahl added: "Incidentally, I dedicated two of my books to Olivia, the first was James and the Giant Peach. That was when she was still alive. The second was The BFG, dedicated to her memory after she had died from measles. You will see her name at the beginning of each of these books. And I know how happy she would be if only she could know that her death had helped to save a good deal of illness and death among other children."
In the letter, Dahl described it as "almost a crime to allow your child to go unimmunised".
His wife Patricia Neal, who was Olivia's mother, said that Dahl was so devastated by Olivia's death that he never spoke about it.
In a recently discovered private notebook kept by Dahl, he wrote about the moment he was told Olivia had died.
"Got to hospital. Walked in. Two doctors advanced on me from waiting room. How is she? I'm afraid it's too late. I went into her room. Sheet was over her. Doctor said to nurse go out. Leave him alone. I kissed her. She was warm. I went out. 'She is warm.' I said to doctors in hall, 'why is she so warm?' 'Of course,' he said. I left."
The contents of the notebook are published in Dahl biography Storyteller.
Dr Ava Easton, Chief Executive of The Encephalitis Society, says that measles encephalitis is still a deadly disease in modern Britain.
"Roald Dahl wrote his letter 30 years ago but still today in the UK alone, 6000 people are diagnosed with encephalitis each year, that's 16 people every day," she tells The Independent. "This, it seems is also considered an underestimate as encephalitis is very difficult to diagnose and like in the case of Roald Dahl's daughter, is sadly often missed.
"On the 22nd of February it is World Encephalitis Day, where we are looking to increase awareness of encephalitis and encourage doctors and the general public to learn more about the condition."
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