Novel filoviruses: indication of a global threat or cause to reassess our risk perception?

Filoviruses such as Ebola virus are widely known as causative agents of severe human disease, although apathogenic filoviruses also exist. There is now increasing evidence that filoviruses circulate in almost all parts of the world, where they are being discovered in an expanding range of sometimes unexpected host species. Here we summarize the current knowledge regarding these novel filoviruses, and open questions that need answering to assess and prepare for the risk they pose.