Walsh & Hoyt: Virus Structure and Classification

The first classification of viruses as a group distinct from other microorganisms was based primarily on the ability of these agents to pass through filters of a small pore size. They thus were called ""filterable agents."" Initial subdivisions were based on pathologic properties, such as specific organ tropism (e.g., gastrointestinal [GI] tropism = enteroviruses) or on common epidemiologic features, such as vector transmission (e.g., arthropod-borne = arboviruses). Increasing knowledge of the morphologic and genetic characteristics of viruses has subsequently permitted their reclassification, using three main components: (a) the type and structure of the viral nucleic acid and the strategy used in its replication, (b) the type of symmetry of the protein coat that surrounds the nucleic acid (the capsid), and (c) the presence or absence of a lipid envelope surrounding the capsid.