美国国家公共电台 NPR How Zika Became So Dangerous For Babies（在线收听）

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Scientists say they have solved a mystery about the Zika virus. They think they've figured out why the virus became a global threat all of a sudden and started causing severe birth defects. As NPR's Michaeleen Doucleff reports, the study shows how quickly viruses can become extremely dangerous.

MICHAELEEN DOUCLEFF, BYLINE: When I received the study, I sent it to a biologist at UC San Diego who's at the forefront of Zika research. His name is Alysson Muotri, and he almost couldn't believe his eyes.

ALYSSON MUOTRI: When you sent it to me, I started reading. I said, oh, my gosh, that's amazing.

DOUCLEFF: Amazing because the study suggests something unexpected. One small change in Zika's genes made it a big threat for pregnant women. For decades, Zika had been a relatively harmless virus that causes only mild symptoms. But then in 2015, it began causing a severe type of birth defect called microcephaly in which babies are born with very small heads. Now researchers at the Chinese Academy of Sciences in Beijing say they know why. Zika mutated. The virus picked up one mutation that dramatically increased its ability to infect fetal brains.

MUOTRI: That single mutation can actually lead to more severe cases of microcephaly, more severe damage in the fetal brain.

DOUCLEFF: This almost sounds like the nightmare scenario, the virus mutating and becoming this very scary thing. Is this common?

MUOTRI: It is common. It happens all the time. Actually, there are several human disorders that's caused by a single mutation. So this happens all the time.

DOUCLEFF: But there are caveats to this study. Hongjun Song is a neuroscientist at the University of Pennsylvania. He says the big caveat is the experiments were performed in mice.

HONGJUN SONG: Mouse is the one model we have now to test the toxicity of the virus in animals, which is the best you can do because we can't really do that in humans.

DOUCLEFF: And the study, which appears in the journal Science, doesn't rule out the possibility that other factors - other mutations or something in the environment - made the virus even worse in South America. Nevertheless, Song says the findings are a bit foreboding.

SONG: I think it's just a little bit scary, right? So basically one unlucky change which happens all the time can cause very big, unlucky events down the road. So I think the lesson, I feel, is we all need to be prepared.

DOUCLEFF: Because the next big outbreak is likely just around the corner. Michaeleen Doucleff, NPR News.

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