Table 1.

Baltimore classification of viruses.

Class	Genomea	mRNA synthesis	Virus families
I	dsDNA	mRNA transcribed directly from DNA template	Herpesviridae Adenoviridae Papillomaviridae Polyomaviridae Poxviridae
II	ssDNA	ssDNA converted to dsDNA before transcription	Anelloviridae Parvoviridae
III	dsRNA	mRNA transcribed from RNA genome	Reoviridae
IV	ssRNA (+)	RNA genome acts as mRNA	Coronaviridae Astroviridae Caliciviridae Faviviridae Picornaviridae Togaviridae
V	ssRNA (−)	mRNA transcribed from RNA genome	Rhabdoviridae Filoviridae Paramyxoviridae Arenaviridae Bunyaviridae Orthomyxoviridae
VI	ssRNA – RT	Genome is converted to dsDNA by RT, it is integrated into the host's genome and then transcribed to mRNA	Retroviridae
VII	dsDNA – RT	DNA is transcribed through an RNA intermediate which acts as mRNA or as template for mRNA synthesis	Hepadnaviridae

^ads = double-stranded; ss = single stranded; RT = reverse transcriptase; + and − indicate strand orientation.