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."
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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