Viral pathogenesis is the study of how viruses cause disease in their hosts. It is a specialized field of study in the science of virology. Pathogenesis is the 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.
Viruses are simple submicroscopic parasites of plants, animals, and bacteria. Some are the causative agents of disease.
Viruses consist essentially of a core of RNA or DNA surrounded by a protein coat. Unable to replicate without a host cell, viruses are typically not considered living organisms.
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.
The host anti-viral defense systems must be ineffective or absent.
The first goal of the invading virus is to successfully enter a host.
The second goal is to multiply and flourish.
The third goal is to disperse to new hosts.
There are several mechanisms that must occur for a viral disease to develop:
Implantation: This is the process by which the virus gains entry into the body. In order to invade, the virus must implant at an entry portal into the body, such as the cells of tissues in the respiratory and gastrointestinal systems, the skin or the genital area.
Replication: To be successful, the invading virus must reproduce itself in large numbers. It usually does this outside of the cells in the fluids around the cells. This replication involves reproduction of the viral DNA and RNA to create new virus particles.
Dispersal: The replicated viruses must spread to specific targeted organs throughout the body. The most common route of spread from the portal of entry to the target organ is the circulatory system, which the virus reaches via the interstitial lymph fluids. From the lymphatic system the viruses enter the blood and are carried to disease sites. Viruses can access target organs from the blood and then by multiplying inside body cells, moving through gaps, or by being carried inside the organ on leukocytes or white blood cells. Some viruses, such as rabies and Herpes viruses, target the nervous system and disseminate through the nerves.
Shedding: In the next step, the viruses must spread to sites where shedding into the environment can occur. The respiratory tracts, the urogenital region and the blood are the most frequent sites of viral shedding.
Factors that affect these pathogenic mechanisms are:
Accessibility: how available the host tissues are to the virus
Susceptibility of the host: how vulnerable the host cells are to virus multiplication
Susceptibility of the virus: whether the virus is vulnerable to the host’s defenses.
How Viral Pathogens Evolve
As with all parasites, natural selection favors the development of low-virulence virus strains that do not kill the host. A living host is far more likely to spread the virus to new hosts than a dead one. Parasites invading a new host species will typically kill large numbers until some immunity and accommodation occurs, after which death rates drop and some hosts are unaffected by the disease but carry it and spread it.