Characteristics and Classification of Viruses

Characteristics of Viruses

Viruses were discovered at the end of the 19th century as filterable agents causing infectious diseases of plants and animals. The International Code of Virus Classification and Nomenclature (ICVCN) defines virus species as follows: “A species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria". The virus consists of a core of nucleic acid and a capsid. Viral genomes can be found as single or double-stranded versions of DNA and RNA. The virus capsid consists of simple helical symmetry and icosahedra forms. A common set of simple forms gives rise to a multitude of complex structures. The size of viruses is relatively small, usually 20-30 nm in diameter. Some viruses have a phospholipid envelope, derived from the infected host’s cell membrane, that surrounds the protein capsid. Virus genomes direct their replication and the synthesis of other viral components, using cellular systems in appropriate host cells. Virus particles are formed by assembly from newly synthesized components within the host cell.

Classification of Viruses

  • International Committee on Taxonomy of Viruses (ICTV) classification
    • Herpesvirales: Large eukaryotic double-stranded DNA (dsDNA) viruses (e.g. Epstein-Barr virus), an icosahedral capsid which encloses the DNA, surrounded by a layer of tegument and an envelope;
    • Caudovirales: tailed dsDNA typically infecting bacteria;
    • Ligamenvirales: linear double-stranded viruses infecting archaea;
    • Mononegavirales: nonsegmented negative (or antisense) strand single-stranded RNA viruses infecting numerous plants, animals, and humans;
    • Nidovirales: positive (or sense) strand single-stranded RNA viruses of vertebrates. Nidovirales possesses the largest known RNA genomes (from 26 to 32kb);
    • Picornavirales: small positive-strand single-stranded RNA viruses infecting plants, insects, and animals;
    • Tymovirales: monopartite positive single-stranded RNA viruses of plants.
  • The Baltimore classification (BC)
    • BC I: viruses with double-stranded DNA genomes that have a replication-expression strategy highly reminiscent of that of cellular organisms;
    • BC II: viruses with single-stranded DNA genomes;
    • BC III: viruses with double-stranded RNA genomes;
    • BC IV: viruses with positive-sense RNA genomes;
    • BC V: viruses with negative-sense RNA genomes;
    • BC VI: viruses with positive-sense RNA genomes that replicate via DNA intermediates produced by reverse transcription of the genome;
    • BC VII (added later to the original system): viruses with double-stranded DNA genomes that package a double-stranded DNA form or an RNA-DNA hybrid into virions and replicate via reverse transcription.
  • Classification based on the structure
    • Euvirus consists of nucleic acid and protein at least.
    • Subvirus
      • Varoid only contains infecting RNA;
      • Virusoid only contains non-infecting RNA;
      • Prion only contains protein.
  • Host domain classification
    • Eukaryotic viruses;
    • Bacterial viruses;
    • Archaeal viruses.

Several viral classification systems. Fig.1 Several viral classification systems. ( Mahmoudabadi, 2018)

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  1. Mahmoudabadi, G.; Phillips, R. A comprehensive and quantitative exploration of thousands of viral genomes. Elife. 2018. 7.

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