In 1962, children's book author Roald Dahl lost his oldest daughter, Olivia, to measles. She was 7 years old.

Twenty-six years later, Dahl wrote a letter to parents about what happened:

"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 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."

Olivia had what's called measles encephalitis. The virus had spread to her brain. Her immune system rushed in to fight it. Her brain swelled up.

"A lot of folks feel measles isn't a big deal. It just causes a rash and a fever," says Dr. Alice Ackerman, chairwoman of the pediatrics department at Virginia Tech Carilion School of Medicine. "In the majority of cases, that's true."

But in about 1 in 1,000 cases, the infection becomes systemic and moves to the brain.

"This causes behavioral changes, potential brain swelling and convulsions," Ackerman says. There's little doctors can do. Children can be left deaf, blind or with mental retardation — if they recover.

Even after a mild, "normal" case of measles, Ackerman says, there's a very small chance brain damage could develop years later.

"It doesn't matter how bad your initial measles infection was," she says. "Up to about eight years later, children can start showing behavioral changes. They may have problems sleeping or start acting funny. It's a degenerative process: The brain gets more and more physically damaged [from inflammation] over time."

"It's heartbreaking," Ackerman says. "I've personally seen about about 10 children in my career with subacute sclerosing panencephalitis, or SSPE. It's very scary. You just kind of say, 'I can't treat this, once this damage is done.' "

SSPE is very rare: Only about 1 in 5,000 people with the measles will develop it, a study found. But other complications from measles aren't so rare.

"As this outbreak here [in the U.S.] progresses, I wouldn't be surprised if we don't see some cases with severe complications," says Dr. Amesh Adalja, an infectious disease specialist at the University of Pittsburgh Medical Center. "One in four cases, so far, have been hospitalized."

In terms of serious problems, the most common is pneumonia.

Measles typically infects the upper respiratory tract, like the flu or common cold does. But when measles moves to the lower respiratory, it becomes pneumonia — and much more problematic.

"Pneumonia impairs the body's ability to harness oxygen in the air," Adalja says. "Children don't get enough oxygen in their lungs to supply the body. Some end up on a ventilator."

Up to 1 in 20 children worldwide who catch measles gets pneumonia, the Centers for Disease Control and Prevention says. It's the most common way that measles kills 145,000 children worldwide.

And there's no way to predict which children will develop severe complications to measles, Adalja says. "For all [intents and] purposes, anyone can get pneumonia from the measles or encephalitis."

But people with weak immune systems are at higher risk, Adalja says. That includes those undergoing some cancer treatments, organ donor recipients, the elderly and babies.

Malnourished children in poor countries are also vulnerable to measles encephalitis and pneumonia, says Virginia Tech's Ackerman. "Their immune systems can't build an appropriate response against the virus."

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