Last year, Sheri Fink, a reporter for The New York Times, spent weeks reporting in West Africa, trying to find out where the Ebola outbreak came from, and how it spread.

During her investigation, she uncovered new clues about how Ebola made its way from Guinea into neighboring Sierra Leone. Experts had initially concluded that the disease spread rapidly there after a funeral for a popular traditional healer named Mendinor. But then Fink learned of another woman, Sia Wanda, who had become sick long before Mendinor died.

Fink’s findings raised an important question: Could there have been a chance to stop the disease before it claimed more than 10,000 lives?

Follow Fink’s investigation in the video below, and read her latest story on scientists’ efforts to map the spread of the virus here.

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