On July 30, she implored Chan to declare an international health emergency. Chan responded that she was being very pessimistic, Liu said.

Liu replied: "Dr. Chan, I'm not being pessimistic. I'm being realistic."

Chan soon flew to West Africa to meet with the presidents of Guinea, Liberia and Sierra Leone, and announced a $100 million push to stop the outbreak.

On Aug. 8, the WHO declared a global health emergency.

Chan declined to comment for this article. The WHO's Fukuda said that if anyone asks whether his organization did a perfect job, the answer will be, "Hell no."

But after six trips to Africa during the epidemic, he has seen a more profound truth: Global organizations can provide epidemiologists and laboratory help, but what these resource-poor countries really need are front-line doctors and nurses, and basic resources. In Africa, patients told him, "We don't have enough food."

He visited a clinic where 25 health-care workers became sick with Ebola and 23 died. Doctors kept going to work even as they were ostracized back home by fearful neighbors. "This is really a profound level of heroism," Fukuda said.

In a sign of ebbing confidence in the WHO's ability to coordinate a response, U.N. Secretary General Ban Ki-moon on Aug. 12 appointed David Nabarro, 65, a longtime troubleshooter, as senior U.N. system coordinator for Ebola. Nabarro had worked on avian flu and the aftermath of the 2004 Indian Ocean tsunami. He was vacationing with his family at a beach in Kenya when he received the call asking him to jump into the crisis.

Over the next month, Nabarro would travel to 21 cities on three continents, trying to put together a coalition and showing everyone an ominous chart depicting four possible trajectories for the epidemic. The best-case scenario showed it ending in the middle of next year. The worst case showed the "epi curve" rising in the wrong direction, toward the vertical, toward an unimaginable catastrophe.