Viruses are named scientifically similarly to organisms; they are referred to by genus and species name. Above these, two classification systems, supplemental to each, create a taxonomy for viruses.
The International Committee on the Taxonomy of Viruses organizes viruses to one of seven orders, which contain families and sub-families prior to the genus and species designations. Viruses are challenging to classify above the taxonomy of an order, as they have relatively small genomes and a high rate of mutation. The Baltimore classification system, first defined in 1971, separates viruses into seven groups based on their combination of nucleic acid into DNA or RNA, whether they are single- or double-stranded, method of replication and polarity of the nucleic acid into positive or negative.