Virus species

A definition of this taxonomic level has proven troublesome, and the ICTV currently defines a virus species as ‘a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche’. Essentially, classification at the level of virus species cannot rely on any single property, but on a combination of shared characteristics. Assigning viruses to a final division that of virus strain is likewise difficult and generally reflects the sharing of distinctive characteristics that are too minor to merit distinction at the species level.