Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. They are as follows:
- Classification based on nucleic acid
- Classification based on structure or symmetry
- Classification based on replication properties and site of replication
- Classification based on host range
- Classification based on mode of transmission
1. Classification based on Nucleic Acid:
(a) DNA virus: The viral genome is DNA.
(i) Double-stranded DNA virus: Examples: Adenovirus,
Herpesvirus.
(ii) Single-stranded DNA virus: Example: Parvovirus.
(b) RNA virus: Genome is RNA.
(i) Double-stranded RNA virus: Example: Reovirus.
(ii) Enveloped Retroviruses Single stranded RNA virus: These
are further classified into two groups.
• Positive sense RNA (+RNA): Polio virus, Influenza virus.
• Negative sense RNA (−RNA): Rabies virus.
2. Classification of Viruses based on Structure:
(a) Cubical virus: They are also known as icosahedral
symmetry virus. Examples: Reo virus, Picorna virus.
(b) Spiral virus: They are also known as helical
symmetry virus. Examples: Paramyxovirus, orthomyxovirus.
(c) Radial symmetry virus: They look like spoke or
wheel type. Example: Bacteriophage.
(d) Complex virus: They possess a capsid that is
neither purely helical nor purely icosahedral. Example: Pox virus.
3. Classification of virus based on replication
properties and site of replication:
(a) Replication and assembly in the cytoplasm of the host:
Example: All RNA viruses replicate and assemble in the cytoplasm of the host
cell except the Influenza virus.
(b) Replication in nucleus and assembly in the cytoplasm
of host: Examples: Influenza virus, Pox virus.
(c) Replication and assembly in the nucleus of the host:
All DNA viruses replicate and assemble in the nucleus of the host cell except the
Pox virus.
(d) Virus replication through ds DNA intermediate:
Examples: All DNA viruses, Retrovirus, and some tumor-causing RNA virus
replicates through ds DNA as intermediates.
(e) Virus replication through ss RNA intermediate:
Example: All RNA viruses except Reovirus and tumor-causing RNA viruses.
4. Classification of Viruses based on Host Range:
(a) Bacteriophage: Phages are viruses infecting
bacteria. Examples: λ phage, T2, T4, φ174, MV-11.
(b) Plant virus: Those viruses that infect plants.
Examples: TMV, cauliflower mosaic virus.
(c) Animal virus: Those viruses that infect animals.
Examples: Poliovirus, Retrovirus, Herpes virus, Adenovirus.
(d) Insect virus: Virus that infects insects.
Examples: Baculovirus, Sacbrood virus, Entomopox virus, Granulosis virus.
5. Classification of virus based on mode of transmission:
(a) Virus transmitted through respiratory route:
Examples: Swine flu, Rhinovirus.
(b) Virus transmitted through fecal-oral route:
Examples: Hepatitis A virus, Poliovirus, Rotavirus.
(c) Virus transmitted through sexual contacts:
Example: Retrovirus.
(d) Virus transmitted through blood transfusion:
Examples: Hepatitis B virus, HIV
(e) Zoonotic virus: Virus transmitted through biting
of infected animals. Examples: Rabies virus, Alphavirus, Flavivirus.