Unlike other recent viral emergences, which were in majority caused by RNA viruses, the monkeypox results from infection by a DNA virus, an orthopoxvirus closely related to both vaccine and smallpox viruses and whose two genomic variants are known. Unexpectedly isolated from captive Asiatic monkeys and first considered as an laboratory curiosity, this virus was recognised in 1970 as an human pathogen in tropical Africa. Here it was responsible for sporadic cases following intrusions (for hunting) into tropical rain forests or rare outbreak with human-to-human transmission as observed in 1996 in Democratic Republic of Congo. As monkeypox in humans is not distinguishable from smallpox (a disease globally eradicated in 1977) it was only subjected to vigilant epidemiological surveillance and not considered as a potential threat outside Africa. This point of view radically changed in 2003 when monkeypox was introduced in the USA by African wild rodents and spread to 11 different states of this country. Responsible for 82 infections in American children and adults, this outbreak led to realize the sanitary hazards resulting from international trade of exotic animals and scientific investigations increasing extensively our knowledge of this zoonosis.