As any classification is essentially an arbitrary human invention, it is imperative to have a unique source of classification also for viruses. The International Committee on Taxonomy of Viruses (ICTV), a committee of the Virology Division of the International Union of Microbiological Societies, has been in charge of this task since 1966 and has already published nine Reports updating the status of viral taxonomy. Contrary to other biological entities (vertebrates, invertebrates, plants, fungi and bacteria), viruses are not classified according to a Linnean latinised binomial system but, like these, they are arranged into species, genera, subfamilies, families and orders, whose names are written in italics with an initial capital letter. The last ICTV report (2011) lists 6 orders, 94 families, 22 subfamilies, 395 genera and 2480 recognised species. Species are the building blocks of the taxonomic construction. Although species identification is based on discriminating traits, their allocation into higher taxa takes into account common characters of increasingly hierarchical significance.

Key Concepts:

• Taxonomy is an arbitrary organisation of biological entities to facilitate scientific studies and to identify common traits within similar taxonomic levels.
  
• Classification and nomenclature are the two facets of taxonomy.
  
• Classification is the arrangement of biological entities into taxonomic categories (taxa) on the basis of similarities and/or relationships (phylogeny).
  
• Nomenclature is the assignment of names to taxa, according to international rules.
  
• Virus classification is a rapidly evolving science particularly with the input of viral genome sequencing.
  
• Viruses are not named according to a Linnean latinised binomial system and instead are named with international English-based names and de novo international taxonomic names.
  
• The ICTV is entrusted with the task of classifying and naming virus taxa.