samples of both humans and animals with or without signs of diarrhea. Bluetongue virus is an economically important pathogen that infects cattle and sheep. In recent years, progress has been made in determining atomic and subnanometer resolution structures of a number of key viral proteins and virion capsids of several dsRNA viruses, highlighting the significant parallels in the structure and replicative processes of many of these viruses. Mutation rates RNA viruses generally have very high mutation rates compared to DNA viruses, because viral RNA polymerases lack the proofreading ability of DNA polymerases. The genetic diversity of RNA viruses is one reason why it is difficult to make effective vaccines against them. Retroviruses also have a high mutation rate even though their DNA intermediate integrates into the host genome (and is thus subject to host DNA proofreading once integrated), because errors during reverse transcription are embedded into both strands of DNA before integration. Some genes of RNA virus are important to the viral replication cycles and mutations are not tolerated. For example, the region of the hepatitis C virus genome that encodes the core protein is highly conserved, because it contains an RNA structure involved in an internal ribosome entry site. Sequence complexity On average, dsRNA viruses show a lower sequence redundancy relative to ssRNA viruses. Contrarily, dsDNA viruses contain the most redundant genome sequences while ssDNA viruses have the least. The sequence complexity of viruses has been shown to be a key characteristic for accurate reference-free viral classification. Replication Animal RNA viruses are classified by the ICTV. There are three distinct groups of RNA viruses depending on their genome and mode of replication: Double-stranded RNA viruses (Group III) contain from one to a dozen different RNA molecules, each coding for one or more viral proteins. Positive-sense ssRNA viruses (Group IV) have their genome directly utilized as mRNA, with host ribosomes translating it into a single protein that is modified by host and viral proteins to form the various proteins needed for replication. One of these includes RNA-dependent RNA polymerase (RNA replicase), which copies the viral RNA to form a double-stranded replicative form. In turn, this dsRNA directs the formation of new viral RNA. Negative-sense ssRNA viruses (Group V) must have their genome copied by an RNA replicase to form positive-sense RNA. This means that the virus must bring along with it the enzyme RNA replicase. The positive-sense RNA molecule then