When the Ebola virus began relentlessly spreading in Sierra Leone months ago, government officials made an urgent plea to Doctors Without Borders, all that appeared to stand between the country and chaos.

“They asked us to be everywhere,” recalled Walter Lorenzi, the medical charity’s former coordinator in Sierra Leone. “They didn’t know what to do.”

Not long after, the group opened a treatment center in Kailahun, in eastern Sierra Leone, that was hacked out of the bush in just 12 days. Before opening another center three weeks ago in the southern city of Bo, the organization ran three shifts of workers, 24 hours a day, when daily rain and equipment breakdowns delayed construction.

The first to respond to the Ebola crisis in West Africa, Doctors Without Borders remains the primary international medical aid group battling the disease there. As local health systems have all but collapsed and most outside institutions, including the United States military, have yet to fulfill all their pledges of help, the charity has erected six treatment centers in West Africa, with plans for more. Its workers have treated the majority of patients, just as they have in previous Ebola outbreaks and some other epidemics in the developing world.