The family Bunyaviridae (named after Bunyamwera, the type virus) consists of over 250 individual virus species grouped into five genera: Bunyavirus, Phlebovirus, Hantavirus, Nairovirus and Uukuvirus. These genera are classifIed in one family primarily because of their stuctural features, including a trisegmented genome of negative polarity and a roughly similar protein-coding pattern within each ge- nome segment (36,344). Biologically and epidemiologically, they encompass a wide range of characteristics, but certain general principles apply to the entire family. Some of the viruses, like those included in the California serogroup (genus Bunyavirus) , Rift Valley fever virus (genus Phlebovirus), Hantaan virus (genus Hantavirus), and Crimean-Congo hemorrhagic fever (genus Nairovirus), are important causes of human disease. However, most bunyaviruses were isolated by collecting large numbers of arthropods, triturating and inoculating them into mice or cell cultures, such isolates rarely infect humans or domestic animals. The bunyaviruses depend on wild animal hosts for thei persistence in nature: human-to-human transmission does not generally occur, and humans are dead-end hosts. Most members of the family are arthropod-borne and cause a persistent, nonlethal, lifelong infection in their hematophagous insect hosts. In nature each individual virus infects a limited number of arthropod and vertebrate hosts, as is true of other arboviruses. There are a few exceptions to most of these generalizations.