Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Added The Baltimore classification is a classification system which groups viruses into families depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds) etc.) and their method of replication.
It was created by the American biologist David Baltimore and is the preferred way of classifying viruses today. Other classifications are determined by the type of disease the virus causes (localised, disseminated, persistent, etc.) or its morphology (spherical, dodecahedral, etc.). Neither are particularly satisfactory because very different viruses will sometimes cause the same disease (e.g. hepatitis), and viruses with similar structures will often behave very differently (what's more, the precise structure of viruses is often difficult to determine under the microscope).
Classifying viruses according to their genome means that those in a given category will all behave in much the same way, which offers some indication of how to proceed with further research.
- Type I: dsDNA viruses (Herpesviridae, Poxviridae, Adenoviridae and Papovaviridae)
- Type II: ssDNA viruses (Circoviridae and Parvoviridae)
- Type III: dsRNA viruses (Reoviridae and Birnaviridae)
- Type IV: positive sense ssRNA viruses (Astroviridae, Caliciviridae, Coronaviridae, Flaviviridae, Picornaviridae, Arteriviridae and Togaviridae)
- Type V: negative sense ssRNA viruses (Arenaviridae, Orthomyxoviridae, Paramyxoviridae, Bunyaviridae, Filoviridae and Rhabdoviridae)
- Type VI: positive sense ssRNA viruses that replicate through a DNA intermediate (Reverse transcriptase) (Retroviridae)
- Type VII: dsDNA viruses with ssRNA intermediates (Reverse transcriptase)(Hepadnaviridae)
"Virus Taxonomy Portal." (Website.) Viral Bioinformatics Resource Center & Viral Bioinformatics - Canada. Retrieved on 2007-09-27.