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While the Ebola outbreak continues to rage in West Africa, it is also unfolding virtually inside computers. Researchers are studying the dynamics of the epidemic, and policy makers turn to these experts for predictions of how far Ebola might spread. NPR's Nell Greenfieldboyce explains.

NELL GREENFIELDBOYCE, BYLINE: Bryan Lewis works at Virginia Tech. He describes himself as a computational epidemiologist. He creates mathematical simulations of how infected people spread disease.

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BRYAN LEWIS: And then we run it through different scenarios. And since it's a computer program, we can say, what if we're able to isolate them better once they were identified, or what if we could identify more of them and see how that affects the overall flow of the disease through the population?

GREENFIELDBOYCE: A few weeks ago, a defense agency asked him to model the Ebola outbreak. So he started plugging data into his computer, like the official numbers on how many people have died or gotten infected. Those are probably underestimates, and Lewis says they really don't have a handle on other important stuff that's going on, like how many infected people stay at home versus go to a hospital, or how burial practices spread infection.

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LEWIS: Some of those factors are the ones that are hard to measure.

GREENFIELDBOYCE: Once more, they can't assume this will play out like past Ebola epidemics. Those hit smaller populations in more isolated, rural areas. Despite all this uncertainty, Lewis says his models have been able to predict the course of the epidemic so far.

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LEWIS: At the moment, these models - at least for Sierra Leone and Liberia - we aren't putting any mitigating factors. Like we're just letting this thing run unthrottled and they're just surging up. And they've been, unfortunately, accurate in the last couple of weeks in terms of the number of cases coming out.

GREENFIELDBOYCE: He says if you just kept the simulation going on and on, it shows Ebola spreading across the continent. But this scenario he's constructed doesn't include all the public health measures that are ramping up now.

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LEWIS: We know in the real world there are efforts being directed out there, there are resources being allocated. And until we understand that better and can incorporate that into the model, I don't think it's very useful to speculate out past a week or two.

GREENFIELDBOYCE: Some computer simulations focus on the risk of Ebola spreading to other countries. Alessandro Vespignani at Northeastern University creates those using information on air travel and other kinds of transportation. He says Ebola could find its way to African nations like Ghana, Gambia and Senegal.

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ALESSANDRO VESPIGNANI: There is a tangible risk of spreading in the region to other countries, probably in the ballpark of 20 to 30 percent in the next few weeks.

GREENFIELDBOYCE: He notes poor countries might have trouble keeping an imported case from spreading, and the larger this outbreak gets, the harder it will be to contain. So while his model currently suggests the risks of Ebola reaching the U.S. or Europe in the next six weeks or so is very small - just a few percent - that could change if the outbreak in Africa continues to grow.

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VESPIGNANI: We need really there to extinguish the fire so that then it doesn't really become a threat to the world in the next months.

GREENFIELDBOYCE: The unit at the Centers for Disease Control and Prevention that's been modeling the outbreak is directed by Martin Meltzer. He says government officials always ask him the same two questions.

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MARTIN MELTZER: How many people are going to die and when is this going to end?

GREENFIELDBOYCE: He tells them too much is unknown to give any reliable answer. Mostly, the models just illustrate the need for old tried and true methods for disease control, such as quickly identifying and isolating patients.

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MELTZER: Modeling won't stop this disease. We know how to stop this disease, it's fairly simple, and it's a matter of getting sample activities and practices in action in place on the ground.

GREENFIELDBOYCE: That's the struggle now because while it's easy to change a line of computer code and say, reduce a transmission rate by 80 percent, it's a lot harder to do that in the real world. Nell Greenfieldboyce, NPR News.

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