Viral pathogenesis is the study of how biological viruses cause diseases in their target hosts, usually carried out at the cellular or molecular level. It is a specialized field of study in virology. Pathogenesis is a process in which an initial infection becomes a disease. Viral disease is a sum of the effects on the host caused by the replication of the virus and of the host's subsequent immune response.
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Mechanisms of Infection
Three requirements must be satisfied to ensure a successful infection of a host. There must be sufficient virus available to initiate the infection. Cells at the site of infection must be accessible, susceptible, and allow the virus to enter, and the host anti-viral defense systems must be ineffective or absent.
There are several mechanisms that must occur for a viral disease to develop:
Factors that affect these pathogenic mechanisms are:
Evolution
As with all parasites, natural selection favors the development of low-virulence virus strains. When a pathogen first invades a new host species, the hosts have little or no immunity and often suffer high mortality. Those that survive though, do so because they have different genetics that offer them some protection from the new pathogen. These survivors then reproduce and pass on those genes, resulting in lower mortality rates in future generations. There is no advantage to a pathogen to kill the host before disperal to new hosts.