Reoviridae is a family of double-stranded RNA viruses. Member viruses have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented genome within multi-layered capsids. Lack of a lipid envelope has allowed three-dimensional structures of these large complex viruses (diameter ∼60–100 nm) to be obtained, revealing a structural and likely evolutionary relationship to the cystovirus family of bacteriophage. There are currently 97 species in this family, divided among 15 genera in two subfamilies. Reoviruses can affect the gastrointestinal system (such as rotaviruses) and respiratory tract. The name "reo-" is an acronym for "respiratory enteric orphan" viruses. The term "orphan virus" refers to the fact that some