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The classification of viruses is based on chemical and morphologic criteria. The two major components of the virus used in classification are (1) the nucleic acid (its molecular weight and structure) and (2) the capsid (its size and symmetry and whether it is enveloped). A classification scheme based on these factors is presented in Tables 31–1 and 31–2 for DNA and RNA viruses, respectively. This scheme was simplified from the complete classification to emphasize organisms of medical importance. Only the virus families are listed; subfamilies are described in the chapter on the specific virus. Figures 31–1 and 31–2 show a classification outline for DNA viruses and RNA viruses, respectively, based on the type of genome, the nature of the nucleocapsid, and whether an envelope is present.

TABLE 31–1Classification of DNA Viruses
TABLE 31–2Classification of RNA Viruses

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