Nurses working at the Ebola holding center at the Ganta Hospital wait in line Oct. 17 to receive rice in lieu of salary payment in Ganta, Liberia. The staff was waiting for the arrival of an Ebola Treatment Unit to accommodate the patients in the city. (Photo by Tanya Bindra for The Washington Post)

World Health Organization officials on Wednesday said they see “glimmers of hope” in Liberia, the epicenter of the Ebola epidemic, with strong evidence that the rate of new cases is declining for the first time since the crisis began.

Bruce Aylward, the WHO’s assistant director-general in charge of the operational response, said during a news conference in Geneva that he is both cautiously optimistic and terrified about the news.

“It’s like saying your pet tiger is under control,” Aylward said. “This is a very, very dangerous disease.” He explained that “the danger now is that instead of a steady downward trend we end up with an oscillating trend where the virus goes up and down” because people let down their guard and areas that were becoming clear of the virus become reinfected.

Aylward said the recent decline in new cases “is real” and is confirmed not only by the scores of empty beds in many clinics, but also by a decrease in laboratory confirmed cases and in burials. As recently as a week ago, local officials expressed alarm that the virus was spreading exponentially. But Aylward said it appears that strategies such as tracing and monitoring those who had come into contact with Ebola patients, the adoption of safe burial practices and an aggressive public awareness campaign were working.

The important takeaway, he said, is that the “classic Ebola control strategies can work at  scale.”

WHO officials also released new numbers about spread of the disease globally. The health organization has confirmed 13,703 confirmed cases so far, up from 9,936 on Oct. 19.

For many weeks now, a truism of the Ebola epidemic is that it moves on virus time while the world moves on bureaucratic time. Now there’s a twist on that phenomenon: The virus is shifting locations, fading in some places while flaring in others, and that is challenging global health officials to find a way to be more nimble in their response.

The most dramatic change has been in Liberia, where there has been a pronounced drop in officially recorded Ebola cases in recent weeks even as Army soldiers are just now flying in by the hundreds and preparing to build 17 new Ebola Treatment Units. The U.S. military is focused exclusively on Liberia and on carrying out a mission developed by Army planners in Germany in September.

On Wednesday, Aylward said those empty beds are still “absolutely” needed and that there were no plans to reduce capacity at this time. He said it’s normal for resources to be shuffled around from one hot zone to another as an epidemic spreads but that it’s too soon to let down defenses.

“These are wily viruses. They are waiting for you to make that kind of mistake,” he said.

Here's how the virus spreads and how contact tracing works to stop outbreaks. (Gillian Brockell/The Washington Post)

Meanwhile the epidemic is raging next door in Sierra Leone with limited signs of abating. Guinea continues to have a high rate of transmission, as well, particularly in the capital of Conokry, according to the World Health Organization.

WHO officials said in a news conference a few weeks ago that without significant progress against the epidemic, there could be 5,000 to 10,000 new cases per week in West Africa by the beginning of December. That dire outlook was consistent with other forecasts by epidemiologists who have warned that there would be catastrophically high numbers of Ebola cases without a robust response.

The Post reported Tuesday that the administrator of the U.S. Agency for International Development believes the situation has improved in Liberia, because there is no longer an exponential increase in new infections. But he framed those remarks cautiously and noted the lack of progress in Sierra Leone:

“I don’t think we’ve turned the corner, but it’s better than exponential growth of new cases,” Rajiv Shah, the administrator of the U.S. Agency for International Development, told The Washington Post after a recent trip to the region. But, he added, “In Sierra Leone we are not yet seeing the kind of trend shift that we’re seeing in Liberia. There’s an urgent need to scale up the effort in Sierra Leone.”

Helen Epstein, a writer specializing in public health who has been in Liberia recently, published an online article Monday in the New York Review of Books saying the decline in new cases there is dramatic, and suggesting that “many Liberians, who at first denied the epidemic was real, have come to their senses and changed their behavior by avoiding direct physical contact with sick or dead people.”

But there is still great uncertainty about the possible reach of the virus into remote areas and rain forests where disease surveillance has been limited. The official WHO statistics show minimal progress, and disease modelers continue to be concerned.

“Hard to know what’s really going on,” said Joshua Michaud, associate director for Global Health Policy at the Kaiser Family Foundation. “Maybe it’s only people looking for good news in an ongoing crisis, maybe reporting is worse than ever and we just don’t have good understanding, maybe people are staying away from the treatment units and cases aren’t being counted, or maybe community behaviors actually have changed in the face of the mounting toll to the point where transmission has been reduced.”

The virus has a history of subsiding, then flaring up again. For example, in Kenema and Kailahun districts in Sierra Leone, new cases dropped to zero for several weeks, but then more recently the districts had 23 and 10 cases, respectively, in a single week, the WHO reported.

Lena H. Sun contributed to this report.