4.2 CLASSIFICATION ON THE BASIS OF HOST ORGANISM

 An alternative approach has been to group viruses according to the host that they infect. This has the attraction that it emphasizes the parasitic nature of the virus–host interaction. However, there are several difficulties with this approach. This form of classification implies a fixed, unchanging, link between the virus and host in question. Some viruses are very restricted in their host range, infecting only one species, such as hepatitis B virus infecting humans, and so a designation based on the host is appropriate. However, others may infect a small range of hosts, such as poliovirus which can infect various primates, and the designation here must reflect this rather than name a single species. The most serious difficulty arises with viruses which infect and replicate within very different species. This can be seen with certain viruses which can infect and replicate within both plants and insects. Designation of a virus by the host it infects is therefore not always straightforward. Overriding all of these difficulties is the problem that even if a number of viruses infect a single species, this characteristic does not imply any other similarities in terms of disease or genetic makeup of the various viruses. A different level of sophistication in terms of defining the host has been used for some viruses, notably the herpesviruses. Having shown that herpesviruses are similar in a number of ways, a classification which defined them in terms of the nature of the host cell they infected, for example with gammaherpesviruses infecting lymphoid cells in the host animal, was described. With the discovery of new herpesviruses which infect lymphoid cells but share characteristics with the other, nongammaherpesviruses, this definition is no longer sustainable. Consequently, studying a single virus which infects a single species or group of species, or indeed a virus which infects a particular cell type, tells us nothing about the fundamental nature of the potentially many other viruses which also infect that host.


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