In the WHO paper, researchers presented their first, full statistical snapshot of the epidemic, which gained a foothold in December of last year and has fallen hardest on Guinea, Liberia and Sierra Leone.

The majority of patients stricken by Ebola are 15 to 44 years old, with men and women suffering at a nearly equal rates, according to researchers. The case fatality rate -- based on confirmed cases only -- is 70.8%, with a slightly lower chance of death -- 64.3% -- for those admitted to a hospital.

Although the epidemic has already killed more people than all previous outbreaks combined, researchers said the virus now wreaking havoc on some of the world’s poorest nations is no more lethal or virulent than other strains.

Instead, its rapid spread is due to "insufficient" control efforts, and a "large intermixing" population that has transported the virus across borders and between rural and urban areas.

The current outbreak has caused more cases and deaths than any previous EVD epidemic (see graph in the Supplementary Appendix). It appears to have started in the Guéckédou district of Guinea. The first case was recorded in December 2013, but that case was probably not the first in this outbreak.1,2 Until the end of April 2014, most cases were reported from Guinea, with a small number in bordering parts of Liberia and Sierra Leone (see graph). In late April, a dip in reported cases in Guinea gave hope that the epidemic was beginning to subside and could be confined largely to one country. That hope was abandoned as the number of confirmed cases in Liberia and Sierra Leone rose sharply during May. By August 16, the cumulative number of confirmed, probable, and suspected cases of EVD in the three worst-affected countries plus Nigeria was 2240, with 1229 deaths. The ratio of deaths to cases implies a case fatality rate of 55%. However, this estimate is approximate, since some cases and deaths (perhaps many) have been missed; in particular, contact tracing in Guinea during the initial period was far from adequate, allowing further opportunities for transmission. Moreover, the fatality rate varies markedly among geographic sites, ranging from 30 to 90% in this epidemic.

Although the largest number of cases was reported in the week starting July 28, the data compiled from Guinea, Liberia, and Sierra Leone give little indication that incidence has begun systematically to decline (see graph). As yet, there is no persuasive evidence that the epidemic is under control. And the recent discovery of cases in Nigeria, which shares no border with Guinea, Liberia, or Sierra Leone, highlights the risk of wider spread across Africa and to other continents. Beyond the immediate health concerns, Ebola is also becoming a humanitarian and economic emergency: schools are being closed, agriculture and mining are under threat as workers leave the affected areas, and cross-border commerce has slowed.