Classification seeks to describe the diversity of<unk> es by naming and grouping them on the basis of similarities. In 1962, André Lwoff, Robert Horne, and Paul Tournier were the first to develop a means of<unk>  classification, based on the Linnaean hierarchical system. This system based classification on phylum, class, order, family, genus, and species.<unk> es were grouped according to their shared properties (not those of their hosts) and the type of nucleic acid forming their genomes. In 1966, the International Committee on Taxonomy of<unk> es (ICTV) was formed. The system proposed by Lwoff, Horne and Tournier was initially not accepted by the ICTV because the small genome size of<unk> es and their high rate of mutation made it difficult to determine their ancestry beyond order. As such, the Baltimore classification system has come to be used to supplement the more traditional hierarchy. Starting in 2018, the ICTV began to acknowledge deeper evolutionary relationships between<unk> es that have been discovered over time and adopted a 15-rank classification system ranging from realm to species. ICTV classification The ICTV developed the current classification system and wrote guidelines that put a greater weight on certain<unk>  properties to maintain family uniformity. A unified taxonomy (a universal system for classifying<unk> es) has been established. Only a small part of the total diversity of<unk> es has been studied. As of 2021, 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 39 classes, 65 orders, 8 suborders, 233 families, 168 subfamilies, 2,606 genera, 84 subgenera, and 10,434 species of<unk> es have been defined by the ICTV. The general taxonomic structure of taxon ranges and the suffixes used in taxonomic names are shown hereafter. As of 2020, the ranks of subrealm, subkingdom, and subclass are unused, whereas all other ranks are in use. :Realm (-viria) ::Subrealm (-vira) :::Kingdom (-virae) ::::Subkingdom (-virites) :::::Phylum (-viricota) ::::::Subphylum (-viricotina) :::::::Class (-viricetes) ::::::::Subclass (-viricetidae) :::::::::Order (-virales) ::::::::::Suborder (-virineae) :::::::::::Family (-viridae) ::::::::::::Subfamily (-virinae) :::::::::::::Genus <unk>  ::::::::::::::Subgenus <unk>  :::::::::::::::Species Baltimore classification The Baltimore Classification of<unk> es is based on the method of viral mRNA synthesis The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern<unk>  classification. The Baltimore classification of<unk> es is based on the mechanism of mRNA production.<unk> es must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each<unk>  family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). In addition, ssRNA<unk> es may be either sense (+) or antisense (−). This classification places<unk> es into seven groups:
Virus
Microorganism
Bacteria
Drug
Drug 
Fungus
Biological agent
Chemical substance