In 1986, Roald Dahl set aside friendly giants, chocolate rivers, and all other whimsical fantasies to talk directly to parents about a real-life tragedy: the loss of his daughter, Olivia.

Olivia had passed away in November 1962 after a bout of measles that caused encephalitis, or brain swelling. She was only 7 years old, and her death left Dahl shocked and bereft; “Roald really almost went crazy,” his wife later told People. But while Olivia’s fatal illness had been devastating, it hadn’t exactly been preventable—a trusted measles vaccine wasn’t available at the time.

By the 1980s, that had changed. As Snopes reports, the UK had recently rolled out a vaccine for measles, mumps, and rubella (MMR), and many people were wary about potential side effects. So Dahl wrote a letter for the Sandwell Health Authority to explain how important it was that parents vaccinate their children.

Dahl began the missive by recounting Olivia’s sudden deterioration. “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. She had steadily improved when, one morning while twisting pipe-cleaners into animal shapes, Dahl realized that “her fingers and her mind were not working together and she couldn’t do anything.” He asked her if she was feeling all right, and she responded that she felt “all sleepy.”

“In an hour, she was unconscious,” he wrote. “In twelve hours she was dead.”

Dahl went on to address the general lack of awareness about how measles could cause deadly conditions like encephalitis, and he also included statistics to show how unlikely it would be for a child to contract harmful side effects from the vaccine.

“I should think there would be more chance of your child choking to death on a chocolate bar than of becoming seriously ill from a measles immunization,” he wrote. “So what on earth are you worrying about? It really is almost a crime to allow your child to go unimmunized.”

You can read the full letter on Roald Dahl’s website here.

[h/t Snopes]