In December 2013, the West African Ebola epidemic began in a village near Guéckédou, a trading town in rural Guinea, but the disease wasn’t identified until February. The Guineans promptly notified health officials in neighboring countries, and in Liberia a team of researchers immediately set out for Lofa County, just over the border from Guéckédou, where a number of mysterious deaths had recently occurred. The Liberians at first assumed the deaths were caused by Lassa fever, a far less deadly disease with symptoms similar to Ebola. Liberia had no lab capable of testing for Ebola then, so the researchers sent some blood samples to France. When the results finally came back in late March showing that Ebola was spreading in Liberia too, “fear grabbed us,” said Tolbert Nyenswah, who now heads the Ebola Task Force in Liberia’s Ministry of Health and Social Welfare.

That first outbreak burned out quickly: there were only twelve documented cases in March and April and then none at all for six weeks. However, on May 29, the disease reappeared in Liberia’s capital, Monrovia, and this time it exploded. There were cases all over the city. Nyenswah and his Liberian colleagues had never dealt with Ebola before, but doctors from the medical charity Doctors Without Borders and the US Centers for Disease Control (CDC), who had fought numerous previous Ebola outbreaks in Central Africa, were on hand to help.

Together, they warned the public through radio announcements, posters, and billboards, and sent health workers to villages throughout the country to tell people to be on the lookout for the disease. They set up a hotline so people could report cases and trained teams of investigators to visit each caller and make a tentative diagnosis based on symptoms. They also equipped a lab to do blood tests locally and built Ebola treatment centers—tent camps where patients could be isolated and receive basic care. They hired over two thousand contact tracers to identify and isolate everyone—such as close relatives of victims and health workers—who might have been exposed to the virus.

This system had worked to control twenty-four previous Ebola outbreaks in sub-Saharan Africa, but it wasn’t working this time. At first five, then ten, then twenty, then forty, then more than sixty cases a day were being reported to the task force. Each infected person had fifteen to twenty “contacts”—usually family members or health workers who might have touched them—and as the epidemic took off in July, half of them went on to develop the disease as well. By early August, the treatment centers were full, the streets were strewn with bodies, and the Ebola hotline…