KELLY MCEVERS, HOST:
Ebola has been spreading in West Africa for more than two years. Each time health officials think they've wiped it out, the virus pops up again. And now scientists think they know why. NPR's Michaeleen Doucleff reports.
MICHAELEEN DOUCLEFF, BYLINE: When Ebola first returned to Liberia last summer, it was like the disease appeared out of thin air. A 17-year-old boy died of Ebola near the capital, and no one could figure out how he caught it. He hadn't been around sick people, hadn't traveled anywhere. So scientists at the U.S. Army Medical Research Institute of Infectious Diseases went hunting for clues.
Jason Ladner and his colleagues isolated Ebola from the teen's blood and sequenced its genome.
JASON LADNER: People were saying that this is probably the most important Ebola genome sequence that has been generated thus far.
DOUCLEFF: Important because they hoped the sequence was going to tell them why Ebola kept coming back. Ladner says the sequence was almost a perfect match with an Ebola genome that had been circulating a year before the boy died which was confusing because during that time, the virus should have changed and mutated. But it didn't. It was like the virus was frozen in time. Ladner says that meant the teen's infection likely came from a dormant virus from a survivor. How is that possible?
LADNER: We were definitely surprised. It's definitely surprising.
DOUCLEFF: Because typically people spread Ebola only when they're sick, not when they're healthy. Daniel Bausch is an infectious disease expert who consults for the World Health Organization. We talked to him over Skype. He says there's been previous evidence that Ebola can linger in some people for many, many months. There was a Scottish nurse who had Ebola in her spinal fluid and around her brain nine months after she had recovered.
DANIEL BAUSCH: Another case of a survivor in the United States had virus in the inside of his eye causing inflammation of the eye.
DOUCLEFF: And Ebola can linger in a man's semen. Bausch says this new case indicates Ebola can stay dormant in a person, reactivate and possibly infect others most likely through sexual contact.
BAUSCH: And that's our number-one concern.
DOUCLEFF: So far, Bausch says, all the new cases in West Africa look like they came from survivors infecting others long after they had recovered. In most instances, sexual contact was likely involved.
BAUSCH: Of course, I'm never going to say that sexual transmission is a good thing, but it's kind of comforting, if you will, to say, OK, well, this is from a mode of transmission that we understand more and more how it happens.
DOUCLEFF: And we know how to stop. Health officials had been worried that Ebola was still spreading out of control somewhere, and they couldn't find it. Now they know that's not the case, and they can focus their efforts on the survivors and sexual transmission. Michaeleen Doucleff, NPR News.
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