Experts are trying to understand how the disease, which has defied the ominous predictions of the world’s top infectious disease researchers, appears to be extinguishing itself with surprising swiftness. In September, the United States Centers for Disease Control and Prevention had projected that, by Jan. 20, the outbreak could reach 1.4 million cases in Liberia and Sierra Leone alone, but by that date only 21,797 were recorded in all three countries.

While many have emphasized the enormous assistance hauled into the region by the United States and international organizations, there is strong evidence, especially here in Monrovia, that the biggest change came from the precautions taken by residents themselves.

“Fundamentally, this is about the extent to which societies change their behaviors, how they change them, and the speed at which they change them,” said Dr. David Nabarro, the United Nations special envoy on Ebola, who made frequent trips to the hot zone at the height of the epidemic. “I believe for various reasons people in Liberia changed quickly and dramatically. I believe Sierra Leoneans changed quickly in some areas and less quickly in some areas.”

When Ebola struck the densely crowded neighborhoods of Monrovia over the summer, the first time a capital city had faced Ebola’s full onslaught, the impact was devastating. Hundreds of new cases appeared around the country every week, hospitals collapsed or overflowed with patients, and sick people lay along the road, sometimes dying before help could reach them.

Disease Watchdogs

Reeling from the explosion of infections in August, volunteer Ebola watchdog groups sprang up in many neighborhoods, typically overseen by local elders and led by educated youths, drawing from a long history of community organizing to survive war, poverty and government neglect.

With little or no outside help in the early months, the groups educated their communities about Ebola, a disease new to this part of Africa, and collected money to set up hand-washing stations at key spots. They kept records of the sick and the dead. Many also placed households under quarantine and restricted visits by outsiders. As the sick were turned away at the gates of treatment centers because of a lack of beds, people inside homes began protecting themselves better, covering their arms in plastic shopping bags as they cared for ailing relatives. The gear became known as Liberian P.P.E.’s, or personal protective equipment, a reference to the more impermeable suits worn by health workers.