The geography of Ebola has shifted, further complicating the efforts to eradicate it: For instance, in Guinea, Ebola is now thought to be in nearly twice as many districts as it was just two months ago, when the United Nations established a new mission to coordinate the international response. And in Sierra Leone, Ebola is ravaging the western part of the country, while only a handful of new cases are surfacing in previous hot spots.

“We need to move like a school of fish or a flock of birds,” said Dr. David Nabarro, the United Nations secretary general’s special envoy for the Ebola response, speaking by telephone from Monrovia, Liberia. “As the situation changes we all need to shift how we respond without a long lag period.”

Last week, the W.H.O. estimated, there were 1,100 new Ebola patients across the three countries most affected by the outbreak — Guinea, Liberia and Sierra Leone — slightly higher than the roughly 1,000 per week at the beginning of October.

But the number of new cases is not rising as fast as it was during the summer.

“We are no longer seeing exponential growth of this and in some areas we are seeing steep decline,” Dr. Aylward said.