Reoviridae is a family of viruses. They have a wide host range, including vertebrates, invertebrates, plants, and fungi. They are unique in that they lack lipid envelopes and package their genomes of discrete double-stranded segments of RNA within multi-layered capsids. Lack of a lipid envelope has allowed threedimensional structures of these large complex viruses (diameter,∼600–1,000) to be obtained. There are currently 87 species in this family, divided among 30 genera. Reoviruses can affect the gastrointestinal system (such as Rotavirus) and respiratory tract. The name "Reo-" is derived from respiratory enteric orphan viruses. The term "orphan virus" refers to the fact that some of these viruses have been observed not associated with any known disease. Even though viruses in the Reoviridae family have more recently been identified with various diseases, the original name is still used.