Classification of Medically Important Viruses

The classification of viruses is based on chemical and morphologic criteria.

The two major components of the virus used in classification are:
(1) The nucleic acid (its molecular weight and structure)
(2) The capsid (its size and symmetry and whether it is enveloped)

DNA viruses
(a) Noneveloped icosahedral virus families:
- Parvoviruses
- Papovaviruses
- Adenoviruses

(b) Enveloped virus families:
- Hepadnaviruses (Enveloped icosahedral viruses)
- Herpesviruses (Enveloped icosahedral viruses)
- Poxviruses (Enveloped complex viruses)


Parvoviruses

Envelope present: No
Capsid symmetry: Icosahedral
Particle size (nm): 22
DNA MW (x10^6): 2
DNA structure: Single-stranded, linear
Medically important viruses:
(i) B19 virus


Papovaviruses

Envelope present: No
Capsid symmetry: Icosahedral
Particle size (nm): 55
DNA MW (x10^6): 3-5
DNA structure: Double-stranded, circular, supercoiled
Medically important viruses:
(i) Papillomavirus


Adenoviruses

Envelope present: No
Capsid symmetry: Icosahedral
Particle size (nm): 75
DNA MW (x10^6): 23
DNA structure: Double-stranded, linear
Medically important viruses:
(i) Adenovirus


Hepadnaviruses

Envelope present: Yes
Capsid symmetry: Icosahedral
Particle size (nm): 42
DNA MW (x10^6): 1.5
DNA structure: Double-stranded, incomplete circular
Medically important viruses:
(i) Hepatitis B virus




Herpesviruses

Envelope present: Yes
Capsid symmetry: Icosahedral
Particle size (nm): 100 (neocapsid; but the envelope varies in size; the entire virus can be as large as 200 nm in diameter)
DNA MW (x10^6): 100-150
DNA structure: Double-stranded, linear
Medically important viruses:
(i) Herpes simplex virus
(ii) Varicella-zoster virus
(iii) Cytomegalovirus
(iv) Epstein-Barr virus


Poxviruses

Envelope present: Yes
Capsid symmetry: Complex
Particle size (nm): 250x400
DNA MW (x10^6): 125-185 
DNA structure: Double-stranded, linear
Medically important viruses:
(i) Smallpox virus
(ii) Vaccinia virus


RNA viruses
(a) Noneveloped icosahedral virus families:
- Picornaviruses
- Caliciviruses
- Reoviruses

(b) Enveloped virus families:
- Flaviviruses (Enveloped icosahedral viruses)
- Togaviruses (Enveloped icosahedral viruses)
- Retroviruses (Enveloped icosahedral viruses)
- Orthomyxoviruses (Enveloped helical viruses)
- Paramyxoviruses (Enveloped helical viruses)
- Rhabdoviruses (Enveloped helical viruses)
- Filoviruses (Enveloped helical viruses)
- Coronaviruses (Enveloped helical viruses)
- Arenaviruses (Enveloped helical viruses)
- Bunyaviruses (Enveloped helical viruses)
- Deltaviruses (Enveloped uncertain viruses)


Picornaviruses

Envelope present: No
Capsid symmetry: Icosahedral
Particle size (nm): 28
RNA MW (x10^6): 2.5
RNA structure: Single-stranded, linear, nonsegmented, positive polarity
Medically important viruses:
(i) Poliovirus
(ii) Rhinovirus
(iii) Hepatitis A virus


Caliciviruses

Envelope present: No
Capsid symmetry: Icosahedral
Particle size (nm): 38
RNA MW (x10^6): 2.7
RNA structure: Single-stranded, linear, nonsegmented, positive polarity
Medically important viruses:
(i) Norwalkvirus
(ii) Hepatitis E virus




Reoviruses

Envelope present: No
Capsid symmetry: Icosahedral
Particle size (nm): 75
RNA MW (x10^6): 15
RNA structure: Double-stranded, linear, 10 segments
Medically important viruses:
(i) Rotavirus


Flaviviruses

Envelope present: Yes
Capsid symmetry: Icosahedral
Particle size (nm): 45
RNA MW (x10^6): 4
RNA structure: Single-stranded, linear, nonsegmented, positive polarity
Medically important viruses:
(i) Yellow fever virus
(ii) Dengue virus
(iii) West Nile virus
(iv) Hepatitis C virus




Togaviruses

Envelope present: Yes
Capsid symmetry: Icosahedral
Particle size (nm): 60
RNA MW (x10^6): 4
RNA structure: Single-stranded, linear, nonsegmented, positive polarity
Medically important viruses:
(i) Rubella virus


Retroviruses

Envelope present: Yes
Capsid symmetry: Icosahedral
Particle size (nm): 100
RNA MW (x10^6): 7 (Contains 2 identical molecules of MW 3.5x10^6)
RNA structure: Single-stranded, linear, 2 segments, positive polarity
Medically important viruses:
(i) HIV
(ii) T-cell leukemia virus




Orthomyxoviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 80-120
RNA MW (x10^6): 4
RNA structure: Single-stranded, linear, 8 segments, negative polarity
Medically important viruses:
(i) Influenza virus


Paramyxoviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 150
RNA MW (x10^6): 6
RNA structure: Single-stranded, linear, nonsegmented, negative polarity
Medically important viruses:
(i) Measles virus
(ii) Mumps virus
(iii) Respiratory syncytial virus


Rhabdoviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 75x180
RNA MW (x10^6): 4
RNA structure: Single-stranded, linear, nonsegmented, negative polarity
Medically important viruses:
(i) Rabies virus




Filoviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 80 (Particles' wide; but can be thousands of nanometers long)
RNA MW (x10^6): 4
RNA structure: Single-stranded, linear, nonsegmented, negative polarity
Medically important viruses:
(i) Ebola virus
(ii) Marburg virus


Coronaviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 100
RNA MW (x10^6): 10
RNA structure: Single-stranded, linear, nonsegmented, positive polarity
Medically important viruses:
(i) Coronavirus




Arenaviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 80-130
RNA MW (x10^6): 5
RNA structure: Single-stranded, circular, 2 segments with cohesive ends, negative polarity
Medically important viruses:
(i) Lymphocytic choriomeningitis virus


Bunyaviruses

Envelope present: Yes
Capsid symmetry: Helical
Particle size (nm): 100
RNA MW (x10^6): 5
RNA structure: Single-stranded, circular, 3 segments with cohesive ends, negative polarity
Medically important viruses:
(i) California encephalitis virus
(ii) Hantavirus


Deltaviruses

Envelope present: Yes
Capsid symmetry: Uncertain (The nucleocapsid appears spherical but its symmetry is unknown)
Particle size (nm): 0.5
RNA MW (x10^6): 5
RNA structure: Single-stranded, circular, closed circle, negative polarity
Medically important viruses:
(i) Hepatitis delta virus