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Infections with virus do not necessarily lead to any or all symptoms of a disease. The severity of such symptoms is a function of the virus genotype, the amount of virus delivered to the host, and the host's general immune competence – the factors involved with virulence of the infection. The same virus in one individual can lead to an infection with such mild symptoms of disease that they are not recognized for what they are, while infection of another individual can lead to severe symptoms.

Generally, a virus infection results in an effective and lasting immune response. This is described in more detail in Part II, Chapter 7; briefly, the host's immune response (already activated by the presence of viral antigens at any and all sites where virus is replicating) reaches its highest level as clinical signs of the disease manifest.

A full immune response to virus infection requires the maturation of B and T lymphocytes. The maturation of lymphocytes results in the production of short‐lived effector T cells, which kill cells expressing foreign antigens on their surfaces. Another class of effector T cells helps in the maturation of effector B cells for the secretion of antiviral antibodies. Such a process takes several days to a week after stimulation with significant levels of viral antigen. An important part of this immune response is the generation of long‐lived memory lymphocytes to protect against future re‐infection.

In addition to the host's immune response, which takes some time to develop, a number of nonspecific host responses to infection aid in limitation of the infection and contribute to virus clearing. Interferons quickly render sensitive cells resistant to virus infection. Therefore, their action limits or interferes with the ability of the virus to generate high yields of infectious material. Other responses include tissue inflammation, macrophage destruction of infected cells, and increases in body temperature, which can result in suboptimal conditions for virus infection.

Table 2.2 Some examples of viral cytopathic effect.

Cytopathic Effect	Features	Virus
Lysis	Lytic infection ultimately results in the loss of integrity of the plasma membrane of the cell.	Enteroviruses
Transformation	Cells lose their requirement for anchorage to a surface during growth. Also lost is contact inhibition. As a result, cells grow over each other and also grow as a suspension culture.	Oncornaviruses
Vacuolization	Proliferation of cytoplasmic vacuoles late during the infectious cycle	Flaviviruses
Cell fusion	Cell–cell fusion leading to the formation of syncytium and polykaryons	Paramyxoviruses
Inclusion bodies	Densely staining structures within the cytoplasm or nucleus of the cell; often indicates location of viral assembly	Various virus families
Apoptosis	Programmed cell death, characterized by nuclear events leading to chromosomal disaggregation	Lentiviruses