But researchers at various universities say that at the virus’s present rate of growth, there could easily be close to 20,000 cases in one month, not in nine. Some of the United States’ leading epidemiologists, with long experience in tracking diseases such as influenza, have been creating computer models of the Ebola epidemic at the request of the National Institutes of Health and the Defense Department…

What worries public health officials most is that the epidemic has begun to grow exponentially in Liberia. In the most recent week reported, Liberia had nearly 400 new cases, almost double the number reported the week before. Another grave concern, the W.H.O. said, is “evidence of substantial underreporting of cases and deaths.” The organization reported on Friday that the number of Ebola cases as of Sept. 7 was 4,366, including 2,218 deaths.

“There has been no indication of any downturn in the epidemic in the three countries that have widespread and intense transmission,” it said, referring to Guinea, Liberia and Sierra Leone.