How are viruses classified?

Viruses may be classified on the basis of various criteria including presence of nucleic acid, chemical composition, viral genome structure, mode of transmission, or mode of replication.

Classification based on transcription of the viral genome or Baltimore Classification 

There are 7 Baltimore groups based on transcription of the viral genome:  

• 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 a RNA intermediate in their life cycle

Classification based on structure or symmetry

Viruses occur in different shapes, from basic helical and icosahedral shapes to more complex shapes. Based on their shape and symmetry, viruses may be classified as: 

1. Radial symmetry viruses 
2. Complex viruses 
3. Rod or Spiral shaped or helical symmetry viruses 
4. Cubical or icosahedral symmetry shaped viruses 

Classification based on the host range

Based on the type of host, viruses may be classified as: 

• Animal viruses
• Plant viruses
• Bacteriophage

Classification based on the mode of transmission

Based on mode of transmission, viruses are classified as: 

• Viruses transmitted through the air to the respiratory tract 
• Viruses transmitted through sexual contact with an infected person
• Viruses transmitted through contaminated food or water
• Viruses transmitted when an infected animal, bird or insect bites a human 
• Viruses transmitted through blood transfusions