Virus taxonomy

Early attempts to classify viruses were based on their pathogenic properties, and the only feature common to many of the  viruses placed together in such groupings was that of organ tropism or disease symptom (e.g. viruses causing hepatitis or respiratory disease). Other more fundamental aspects (e.g. virus structure and composition) led to the con- sensus that these initial attempts at classification were inadequate and in the late 1950s and early 1960s, hundreds of new viruses were isolated, highlighting the need for a robust classification system. In 1966 the International Committee on Nomenclature of Viruses (ICNV) was established at the International Congress of Microbiology held in Moscow. From this beginning, the present classification scheme has evolved, and acceptance of the characteristics to be considered and their respective weighting has become universal.The International Committee on Taxonomy of Viruses or ICTV provides a universal tax- onomy database consisting currently (2010) of 6 orders, comprising a total of 87 families,19 subfamilies and 348 genera. To date, 2288 species of virus have been discovered, but hundreds remain unassigned to any of the taxonomic groupings, largely because of lack of data and the continual emergence of new viruses.

As techniques in serology, microscopy, and molecular biology (particularly nucleotide sequencing) have advanced, taxonomy encompasses more and more characteristics and the ICTV classification scheme is supported by extensive verifiable data. Taxonomic factors (Table 1) now include morphological, physical, and physicochemical properties of the virion, such as genomic, protein, lipid, carbohydrate, antigenic, and biological properties. Actual virus definition requires the determination of several hundred characters, but the rapidity of genome sequencing now makes classification of newly emerging viruses much easier; for example, the virus first associated with severe acute respiratory syndrome (SARS) was identified as a Coronavirus within just a few months of its emergence in 2002.

In parallel with the ICTV taxonomy is the Baltimore classification scheme, which considers the composition of the virus genome and its replication strategy. This simple but elegant system, devised in 1970 by Nobel laureate David Baltimore, defines seven classes of virus, explored more fully in the next section.