18.1.6—Virus classification

Viruses are acellular and have no metabolism of their own, so they depend on host cells for replication. They are therefore outside the cellular three-domain and Eukaryotic-kingdom classifications.

Classify a virus by two genome features:
Nucleic-acid type — DNA or RNA.
Strand form — single-stranded or double-stranded.
These give four broad combinations: single-stranded DNA, double-stranded DNA, single-stranded RNA and double-stranded RNA.

Use the classification in this order:

1. Identify whether the viral genome is DNA or RNA.
2. Determine whether that genome is single-stranded or double-stranded.
3. Report the combination as the virus classification; do not use the host organism to replace the genome evidence.
4. Keep the boundary clear: this is a nucleic-acid classification of an acellular, host-dependent entity, not a domain or kingdom assignment.

A virus is not placed in Bacteria, Archaea, Eukarya or one of the four Eukarya kingdoms simply because of the cell it infects. This card stops at genome-based classification and does not add named virus examples or replication-cycle detail.