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Viral pathogenesis involves complex interactions between viruses and hosts comprising of transmission, replication, dissemination, immune response, and pathology to produce disease in humans. Viruses have found several routes to enter in the host, spread in the host to find a target cell/tissue where they can replicate efficiently, and cause cytopathic effects to damage the tissue. In some cases, the immune system is successful in eliminating the virus, whereas in other cases viruses avoid elimination by the immune system and persist in the host. While in several cases the disease is caused by direct viral lysis of the infected cells, in other cases the disease is immune mediated such as immune complexes and cytokines. Many DNA viruses and some RNA viruses transform cells causing oncogenesis. Host factors and defenses play important roles in viral pathogenesis. It is interesting to note that the same virus may cause a mild disease in some hosts and a severe disease in other hosts. Innate and adaptive immune responses are critical to eliminate or control viral infections in hosts. Several viral infections cause immune suppression, including a risk of opportunistic and super infections. Immunocompromised hosts are vulnerable to many viral diseases. Vaccination is the key to provide protection in the population.

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 and cytopathogenicity, (5) patterns of viral infection and disease, (6) host factors, (7) host defense, and (8) virus-induced immunopathology. The stages of a typical viral infection and its pathogenesis (eg, poliovirus pathogenesis) are shown in Figure 7–1.

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


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.


Viruses are transmitted via horizontal (common route of transmission: person to person), and vertical (mother-to-child transmission) routes or vector transmission (from mosquitoes, ...

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