The human genome contains 20,000-25,000 genes, so, as one might expect, viral genomes are typically much smaller than this. Viral genomes can either be circular or linear, and both formats are susceptible to recombination events.

Different classes of viruses have different nucleic acid genomes. Double-stranded DNA, single-stranded DNA, double-stranded RNA, and single-stranded RNA can all be found in certain virions.