Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause.

The International Committee on Taxonomy of  Viruses (ICTV)

The ICTV began to devise and implement rules for the naming and classification of viruses early in the 1970s, an effort that continues to the present.

The ICTV is the only body charged by the International Union of Microbiological Societies with the task of developing, refining, and maintaining a universal virus taxonomy.

The ICTV viral classification starts at the level of the realm and continues as follows, with the taxonomic suffixes in parentheses

• Realm (-viria)
• Subrealm (-vira)
• Kingdom (-viriae)
• Subkingdom (-virites)
• Phylum (-viricota)
• Subphylum (-viricotina)
• Class (-viricetes)
• Subclass (-viricetidae)
• Order (-virales)
• Suborder (-virineae)
• Family (-viridae)
• Subfamily (-virinae)
• Genus (-virus)
• Subgenus (-virus)
• Species

As of 2020, all levels of taxa except sub realm, subkingdom, and subclass are used.

Up to March 2020 based on ICTV classification, there were;

–6 realms,

–10 kingdoms,

–17 phyla,

–2 subphyla,

–39 classes,

–59 orders,

–8 suborders,

–189 families,

–136 subfamilies,

–2,224 genera,

–70 subgenera,

–9,110 species

Criteria for viral classification

Viral classification is based on:

i.Type of the nucleic acid including the size of the genome, strandedness (single or double), linear or circular, positive or negative (sense), segments (number and size)

ii. Based on the outer covering (presence or absence of envelope)

iii. Symmetry of the protein shell (Capsid)

iv. Strategy of viral replication- nuclear/cytoplasm

v. Based on the host they affect

vi. Based on the site of pathogenicity

1. Based on viral nucleic acid, capsid symmetry, outer covering, strandedness and sense/polarity

Classification of RNA viruses

Classification of DNA Viruses

1. Baltimore Classification of viruses

Baltimore classification is a system used to classify viruses based on their manner of messenger RNA (mRNA) synthesis. By organizing viruses based on their manner of mRNA production, it is possible to study viruses that behave similarly as a distinct group.

Seven Baltimore groups are described that take into consideration whether the viral genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), whether the genome is single- or double-stranded, and whether the sense of a single-stranded RNA genome is positive or negative.

• Group I: double-stranded DNA viruses
• Group II: single-stranded DNA viruses
• Group III: double-stranded RNA viruses
• Group IV: positive-sense single-stranded RNA viruses
• Group V: negative-sense single-stranded RNA viruses
• Group VI: single-stranded RNA viruses with a DNA intermediate in their life cycle
• Group VII: double-stranded DNA viruses with an RNA intermediate in their life cycle

2. Based on the site of pathogenicity

a) Neurotropic viruses

-Viruses affecting the nervous system e.g. Polio and Rabies

b) Dermatotropic viruses

-Viruses producing skin lesions e.g. Small pox, Chicken pox and Measles

c) Pheumatotropic viruses

-Viruses affecting the respiratory tract e. g. Rhino virus

3. Based on the site of replication in the cell

Cytoplasmic (poxviruses) or nucleic viruses (herpesviruses)

Based on the host they affect

a) Phagina

-Viruses infecting bacteria e.g. bacteriophages

b) Phytophagina

-Viruses infecting plants e.g. Tobacco Mosaic virus (TMV), Tobacco necrosis virus (TNV)

c) Zoophagina

-Viruses infecting animals e.g. Pox virus, Herpes Simplex virus