 The objective of this trial is to assess in healthy adults the safety and reactogenicity of a new candidate vaccine, cAd3-EBOZ, made of a chimpanzee Adenovirus vector encoding the glycoprotein of Zaire Ebola virus. The secondary objectives will be to assess the immunogenicity of the candidate vaccine and find the most suitable dose for further deployment in epidemic areas in Africa. The 120 planned study subjects will be composed of possibly exposed volunteers owning to organisations such as Médecins sans frontières and susceptible to be deployed in the outbreak zone named as possibly exposed volunteers. The other volunteers will be adults with no planned travels to the epidemic zone named as not exposed volunteers. The first group will be randomly allocated to two different groups low dose single injection of 2.5x10e10 viral particles vp, high dose single injection of 5x10e10 vp. The second group will be randomly allocated to three different groups low dose single injection of 2.5x10e10 viral particles vp, high dose single injection of 5x10e10 vp or placebo single injection of vaccine diluent. The design will be double-blind. Follow-up visits will take place at Day 1, 7, 14, 28, 90 and 180. 