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.
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:
- Implantation: The virus must implant at the entry portal into the body. Viruses usually implant on cells of respiratory, gastrointestinal, skin and genital tissues.
- Replication: The invading virus must reproduce itself in large numbers. It usually does this extracellularly.
- Dispersal: The replicated viruses must spread to target organs (disease sites) throughout the body. The most common route of spread from the portal of entry is the circulatory system, which the virus reaches via the lymphatic system. Viruses can access target organs from the blood capillaries by multiplying inside endothelial cells, moving through gaps, or by being carried inside the organ on leukocytes. Some viruses, such as Herpes, rabies and polio viruses, can also disseminate via nerves.
- Shedding: The viruses must spread to sites where shedding into the environment can occur. The respiratory, alimentary and urogenital tracts and the blood are the most frequent sites of shedding.
Factors that affect these pathogenic mechanisms are:
- how accessible the host tissues are to the virus,
- how susceptible the host cells are to virus multiplication,
- whether the virus is susceptible to the host's defenses.
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. These is no advantage to a pathogen to kill the host before disperal to new hosts.