[The origin and evolution of viruses].

This paper describes various hypotheses for the origin of viruses, prerequisites for the introduction of a virus into a host population, and the concerted evolution of virus and host. In general, viruses evolve through mutations and genetic recombinations. Selection pressure is exerted by various host defence mechanisms and possible ways of virus transmission. The virus variants showing a high reproduction rate without killing their host are most likely to be selected for. In addition, the viral infection vice versa exert a selection pressure on the host population resulting in the elimination of highly susceptible individuals and an increased number of resistant individuals. The apparent consequence is a tendency for the virus to be less virulent and the host to be more resistant.