Although viruses are not classified as members of the domains discussed in chapter 1, they are diverse enough to require their own classification scheme to aid in their study and identification. In an in formal and general way, we have already begun classifying viruses— as animal, plant, or bacterial viruses; enveloped or naked viruses; DNA or RNA viruses; and helical or icosahedral viruses. These introductory categories are certainly useful in organization and description, but the study of specific viruses requires a more standardized method of nomenclature. For many years, the animal viruses were classified mainly on the basis of their hosts and the kind of diseases they caused. Newer systems for naming viruses also take into account the actual nature of the virus particles themselves, with only partial emphasis on host and disease. The main criteria now used to group viruses are structure, chemical composition, and similarities in genetic makeup, which indicate evolutionary relatedness.
The International Committee on the Taxonomy of Viruses lists 59 orders, 189 families, and 2,224 genera of viruses. Nomenclature of viruses follows these conventions: Virus families are italicized and given the suffix -viridae, and virus genera are likewise italicized and end in -virus.
Historically, some virologists had created an informal species naming system that mirrors the species names in higher organisms, using genus and species epithets such as Measles morbillivirus. The species category has created a lot of controversy within the virology community. Many scientists argue that nonorganisms such as viruses are too changeable and that fine distinctions used for deciding on species classifications will quickly disappear. All the same, virologists have largely accepted the concept of viral species, defining them as distinct virus types that share a collection of properties such as host range, pathogenicity, and genetic makeup. The influenza viruses are an example of how variations in the receptors can give rise to different strains or subtypes, even though they are basically the same viruses in other characteristics.
Because standardized species names are not as widely used, the genus or common English vernacular names (such as poliovirus and rabies virus) predominate in discussions of specific viruses in this text. Table 1 illustrates a system of classification for important viruses and the diseases they cause. To-scale examples of each virus family are included.
Table1. Important Human Virus Families, Genera, Common Names, and Types of Diseases
Table1. Important Human Virus Families, Genera, Common Names, and Types of Diseases (Continued)
Characteristics used for placement in a particular family include type of capsid, nucleic acid strand number, presence and type of envelope, overall viral size, and area of the host cell in which the virus multiplies. Some virus families are named for their microscopic appearance (shape and size). Examples include rhabdoviruses, which have a bullet-shaped envelope, and togaviruses, which have a cloaklike envelope. Anatomical or geographic areas have also been used in naming. For instance, adenoviruses* were first discovered in adenoids (one type of tonsil), and hantaviruses were originally isolated in the Korean Province of Hantaan. Viruses can also be named for their effects on the host. Lentiviruses tend to cause slow, chronic infections. Acronyms made from blending several characteristics include picornaviruses, which are tiny RNA viruses, and hepadnaviruses (for hepatitis and DNA).
