Argentine hemorrhagic fever (AHF) was recognized as a new clinical entity from the richest farming region of Argentina in the 1950s (Arribalzaga 1955). The etiologic agent of this disease, Junin virus (JUN), was isolated in 1958 (Parodi et al. 1958; Piroski et al. 1959). JUN belongs to the Arenaviridae family, which includes other rodent-borne pathogens which are important causes of hemorrhagic fever in Africa, (Lassa) and South America (Machupo, Guanarito, and Sabia viruses). The Arenaviridae comprises at least 20 recognized members. Arenaviruses are enveloped RNA viruses, and are divided into two groups, with low level antigenic relatedness: the Old World group, and New World group or Tacaribe complex. Two antigenic subgroups were defined within the New World arenaviruses. JUN is contained into the first group, together with Amapari, Latino, Machupo and Tacaribe viruses. Phylogenetic analysis have shown that Old World and New World arenaviruses occupied two distinct clades, and that New World arenaviruses comprise three evolutionary lineages, named A, B, and C. JUN is contained in lineage B, together with the other three agents causing South American hemorrhagic fevers (Enria et al. 1999a).