Although the current epidemic of EVD in West Africa is unprecedented in scale, the clinical course of infection and the transmissibility of the virus are similar to those in previous EVD outbreaks. The incubation period, duration of ill- ness, case fatality rate, and R0 are all within the ranges reported for previous EVD epidem- ics.7,13-18 Our estimates of R0 are similar to other recent estimates for this West Africa epidem- ic.19-23 The combination of signs and symptoms recorded between symptom onset and clinical presentation is also similar to that in other re- ports.14,17,24-26 We infer that the present epidemic is exceptionally large, not principally because of the biologic characteristics of the virus, but rath- er because of the attributes of the affected popu- lations and because control efforts have been insufficient to halt the spread of infection.