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Terminology can be confusing. Virologists often use terms such as “strains,” “variants,” and “mutants” to designate a virus that differs in some heritable way from a parental or wild-type virus. In conventional usage, the wild type is defined as the original (often laboratory-adapted) virus from which mutants are selected and which is used as the basis for comparison. A wild-type virus may not be identical to a virus isolated from nature. In fact, the genome of a wild-type virus may include numerous mutations accumulated during propagation in the laboratory. For example, the genome of the first isolate of poliovirus obtained in 1909 undoubtedly is very different from that of the virus we call wild type today. We distinguish carefully between laboratory wild types and new virus isolates from the natural host. The latter are called field isolates or clinical isolates.

The field of viral taxonomy has its own naming conventions which can cause some confusion. Viruses are classified into orders, families, subfamilies, genera, and species. These names are always italicized and start with a capital letter (e.g., Picornaviridae). To ensure clarity, the names of viruses (like poliovirus) should be written differently from the names of species (which are constructs that assist in the cataloging of viruses). A species name is written in italics with the first word beginning with a capital letter (other words should be capitalized if they are proper nouns). For example, the causative agents of poliomyelitis, poliovirus types 1, 2, and 3, are members of the species Enterovirus C. A virus name should never be italicized, even when it includes the name of a host species or genus, and should be written in lowercase: for example, Sida ciliaris golden mosaic virus. A good exercise would be to see how often we have accidentally violated these rules in this textbook.

Figure 3.11 Reassortment of influenza virus RNA segments. (A) Progeny viruses of cells that are coinfected with two influenza virus strains, L and M, include both parents and viruses that derive RNA segments from them. Recombinant R3 has inherited segment 2 from the L strain and the remaining seven segments from the M strain. (B) ³²P-labeled influenza virus RNAs were fractionated in a polyacrylamide gel and detected by autoradiography. Migration differences of parental viral RNAs (M and L) permitted identification of the origin of RNA segments in the progeny virus R3. Panel B reprinted from Racaniello VR, Palese P. 1979. J Virol 29:361–373.