Viruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life", since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolise and require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.
Viral classification starts at the level of order and continues as follows, with the taxon suffixes given in italics:
- Order (-virales)
- Family (-viridae)
- Subfamily (-virinae)
- Genus (-virus)
Species names generally take the form of [Disease] virus.
- Caudovirales are tailed dsDNA (group I) bacteriophages.
- Herpesvirales contain large eukaryotic dsDNA viruses.
- Ligamenvirales contains linear, dsDNA (group I) archaean viruses.
- Mononegavirales include nonsegmented (-) strand ssRNA (Group V) plant and animal viruses.
- Nidovirales are composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts.
- Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect and animal hosts.
- Tymovirales contain monopartite (+) ssRNA viruses that infect plants.
Other variations occur between the orders: Nidovirales, for example, are isolated for their differentiation in expressing structural and nonstructural proteins separately.