How Zika Became So Dangerous For Babies

Scientists have started to unravel a key mystery about the Zika virus. And the findings are almost unbelievable.

"When I first started reading the study, I said, 'Oh, my gosh, that's amazing,' says molecular biologist Alysson Muotri, at the University of California, San Diego, who wasn't involved in the study.

The study — published Thursday in the journal Science — demonstrates how an obscure virus may have transformed into a global threat almost overnight.

For decades, Zika had been a relatively innocuous disease. Since in 1947, the mosquito-borne virus had been circulating around Africa and Asia, almost undetected. It caused only a mild illness — a fever, a rash and joint pain. About 80 percent of people had no symptoms at all. And outbreaks tended to be small.