Since 1994, there have been nine human Ebola-Zaire virus (EBOV) outbreaks in eastern Gabon and northwestern Congo [1–3]. A majority of them originated from the handling of ape carcasses found by local hunters [4]. The impact of Ebola-Zaire virus on great ape density is suspected to be high [2,5,6], but neither the demographic consequences of outbreaks nor the way the virus spreads within an ape population are well known. The large population of western lowland gorillas, Gorilla gorilla gorilla, monitored since 2001 at the Lokoué clearing, Odzala-Kokoua National Park, Congo, was affected in 2004, providing us with the opportunity to address both questions using an original statistical approach mixing capture–recapture and epidemiological models. The social structure of gorillas strongly influenced the spread of EBOV. Individuals living in groups appeared to be more susceptible than solitary males, with respective death rates of 97% and 77%. The outbreak lasted for around a year, during which gorilla social units (group or solitaries) got infected either directly from a reservoir or from contaminated individuals.