Author Roald Dahl (Charlie and the Chocolate Factory, James and the Giant Peach and Matilda), lost his 7-year-old daughter Olivia to measles in 1962.

Twenty-four years later, he wrote an essay about her death as a plea to parents everywhere to vaccinate their children. In the wake of the U.S.’s recent measles outbreak – largely attributed to parents consciously deciding not to have their children vaccinated – Dahl’s letter (published in a 1988 pamphlet from the Sandwell Health Authority) has resurfaced as a powerful reminder that measles is largely a preventable disease and that ignoring vaccinations can have devastating effects.

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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. 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 colored 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’s daughter died when her measles progressed to an untreatable disease called measles encephalitis. In 1962, no vaccine existed for measles, but by 1986, one had been developed and Dahl advocated for its use. “It really is almost a crime to allow your child to go unimmunized,” he wrote.

Dahl dedicated two of his books to Olivia, James and the Giant Peach and The BFG, but his wife Patricia Neal, Olivia’s mother, said Dahl never spoke about their daughter’s death.

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.

Dahl’s full essay is available to read on his website. You can also find links to read about the children’s charity established in his memory in 1990 or donate to it. “Roald Dahl wrote his letter 30 years ago but still today in the U.K. alone, 6,000 people are diagnosed with encephalitis each year. That’s 16 people every day. 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,” notes Dr. Ava Easton of The Encephalitis Society, who adds that February 22 is World Encephalitis Day, designed to spread awareness of the disease.