Introduction. The term ‘pestivirus’ was coined in 1973 to group together two antigenically related enveloped RNA viruses: hog cholera virus (HCV) and bovine viral diarrhoea virus (BVDV; Horzinek, 1973). A third animal pathogen, the border disease virus (BDV) of sheep, was found later to be a close relative of BVDV. Pestiviruses are among the smallest enveloped animal RNA viruses (about 40 nm in diameter) and possess a nucleocapsid of non-helical, probably icosahedral symmetry (Horzinek et al., 1967); they share these traits with the numerous flaviviruses, of which the arthropod-borne yellow fever virus is the prototype. The pestiviruses are not arthropod-borne and currently hold generic status in the family Togaviridae. Previously, flaviviruses also held generic status in this family. However, when details of flavivirus molecular structure, replication strategy and gene sequence became known in the early 1980s, the Togaviridae Study Group recognized the fundamental differences and proposed the creation of the new family Flaviviridae with Flavivirus as the only genus (Westaway et al., 1985).