Viruses are ideal objects for studying evolutionary processes because of their short generation time, high numbers of offspring that they produce during infection and not least because of their simple structure. Viruses must continuously adapt to the conditions of their host or their host populations, so selection mechanisms are accessible to experimental approaches. In this context, different criteria play an important role, such as the antigenic diversity, the extent of virus excretion, and the degree of virulence. The complete adaptation of a virus to its host, which leads to a minimization of virulence of the infectious agent, is for both parties the desirable consequence: i.e. a problem-free coexistence and survival. For example, hepatitis G virus (GB virus C) which was initially isolated from patients with liver inflammation, seems to persist in many people without causing illnesses. A similar situation is observed with torque teno viruses ( Sects. 14.5 and 20.2). Spumaviruses are also found in many animal species and humans without causing symptomatic infections ( Sect. 18.1). For many viruses, the maximum exploitation of genetic variability is not always useful. Viruses reach a limit at which a greater variance is no longer advantageous: the proportion of non-infectious virus variants among the progeny becomes too high, whereby the potentially possible error limit is reached.
KeywordsAvian Influenza H5N1 Virus Simian Immunodeficiency Virus Bovine Spongiform Encephalopathy Torque Teno Virus
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