Viruses can be classified in several ways. The most useful is probably by the type of nucleic acid the virus contains, but they can also be classified:
- by their geometry
- by whether they have envelopes
- by the identity of the host organism they can infect
- by mode of transmission
- by the type of disease they cause
Classification by type of constituent nucleic acid (i.e. by genome type), with examples:
- Class I double stranded DNA.
- Class II single stranded DNA.
- Parvovirus (most depend on coinfection with adenoviruses for growth)
- Class III double stranded RNA.
- Class IV positive single stranded RNA itself acting as mRNA.
- Class V negative single stranded RNA used as a template for mRNA synthesis.
- Class VI positive single stranded RNA with a DNA intermediate in replication.
- Class VII double stranded DNA with an RNA intermediate in replication.
