Viral pathogenesis is the process by which viruses produce disease in the host. The factors that determine the viral transmission, multiplication, dissemination, and development of disease in the host involve complex and dynamic interactions between the virus and the susceptible host. Viruses cause disease when they breach the host's primary physical and natural protective barriers; evade local, tissue, and immune defenses; spread in the body; and destroy cells either directly or via bystander immune and inflammatory responses. Viral pathogenesis comprises of several stages, including (1) transmission and entry of the virus into the host, (2) spread in the host, (3) tropism, (4) virulence, (5) patterns of viral infection and disease, (6) host factors, (7) and host defense. The stages of a typical viral infection and its pathogenesis (eg, poliovirus pathogenesis) are shown in Figure 7–1.
Stages of poliovirus pathogenesis.The diagram illustrates multiple steps of poliovirus pathogenesis, starting from virus entry through oropharynx (fecal–oral transmission), virus multiplication at the site of entry (gut), invasion of the virus to the regional lymph nodes, development of viremia, virus shed in feces, virus crossing the blood–brain barrier, virus replication in anterior horn cells, cell destruction, motor neurons are damaged, and development of paralysis.
The process by which viruses cause disease in the host is called viral pathogenesis
Complex interactions between the virus and susceptible host result in disease
An important aspect of viral pathogenesis involves viral epidemiology, enabling physicians to study the distribution and determinants of disease in human populations. Understanding factors that influence acquisition and spread of infectious disease are essential for developing methods of prevention and control. Infection in a population can be endemic (disease present at fairly low, but constant, level), epidemic (infection greater than normally occurs in the population), or pandemic (infections that are spread worldwide involving a novel virus and person-to-person spread). Infection can be direct, for instance, respiratory spread of influenza virus, or indirect, for example, arboviruses (West Nile virus, yellow fever virus, Dengue virus) transmission involving a mosquito vector
Epidemiology deals with distribution and determinants of disease in human populations
Understanding the distribution and spread of disease involves several epidemiologic measures. Infectivity is the frequency with which an infection is transmitted when there is contact between a virus and a susceptible host, and represents the ability of the virus to infect an individual. Measures of infectivity are generally expressed as attack rates (number of persons infected after exposure/the number of susceptible persons). Disease index or pathogenicity is the ability of a pathogen to produce infection and cause disease (number of persons with clinical disease/total number infected). Virulence is a measure of severity of disease when infection occurs and represents the degree of damage done by a pathogen (number of persons with fatal or severe disease/total number infected). Incidence...