The Baltimore classification system is an alternative to the ICTV nomenclature that categorizes viruses into seven groups based on their genome characteristics (such as DNA or RNA, single- or double-stranded) and replication strategies. Viruses in each of these seven categories share common genetics and biology. The groups are:

1. Double-stranded DNA (dsDNA) viruses (e.g., Adenoviruses, Herpesviruses, Poxviruses)
2. Single-stranded DNA (ssDNA) viruses (e.g., Parvoviruses)
3. Double-stranded RNA (dsRNA) viruses (e.g., Reoviruses)
4. Positive-sense single-stranded RNA ((+)ssRNA) viruses (e.g., Coronaviruses, Picornaviruses, Togaviruses)
5. Negative-sense single-stranded RNA ((−)ssRNA) viruses (e.g., Orthomyxoviruses, Rhabdoviruses)
6. Single-stranded RNA reverse-transcribing (ssRNA-RT) viruses with a DNA intermediate (e.g., Retroviruses)
7. Double-stranded DNA reverse-transcribing (dsDNA-RT) viruses with an RNA intermediate (e.g., Hepadnaviruses)