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The fundamental process of viral infection is the viral replicative cycle. The cellular response to that infection may range from no apparent effect to cytopathology with accompanying cell death to hyperplasia or cancer.

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Viral disease is some harmful abnormality that results from viral infection of the host organism. Clinical disease in a host consists of overt signs and symptoms. A syndrome is a specific group of signs and symptoms. Viral infections that fail to produce any symptoms in the host are said to be inapparent (subclinical). In fact, most viral infections do not result in the production of disease (Figure 30-1).

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Figure 30-1
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Types of host and cellular responses to virus infection. (Modified with permission from Evans AS: Epidemiological concepts. In Evans AS, Brachman PS [editors]: Bacterial Infections of Humans, 3rd ed. Plenum, 1998. With kind permission of Springer Science+Business Media.)

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Important principles that pertain to viral disease include the following: (1) many viral infections are subclinical; (2) the same disease may be produced by a variety of viruses; (3) the same virus may produce a variety of diseases; (4) the disease produced bears no relationship to viral morphology; and (5) the outcome in any particular case is determined by both viral and host factors and is influenced by the genetics of each.

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Viral pathogenesis is the process that occurs when a virus infects a host. Disease pathogenesis is a subset of events during an infection that results in disease manifestation in the host. A virus is pathogenic for a particular host if it can infect and cause signs of disease in that host. A strain of a certain virus is more virulent than another strain if it commonly produces more severe disease in a susceptible host. Viral virulence in intact animals should not be confused with cytopathogenicity for cultured cells; viruses highly cytocidal in vitro may be harmless in vivo, and, conversely, noncytocidal viruses may cause severe disease.

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Important features of two general categories of acute viral diseases (local, systemic) are compared in Table 30-1.

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Table 30-1 Important Features of Acute Viral Diseases
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To produce disease, viruses must enter a host, come in contact with susceptible cells, replicate, and produce cell injury. Understanding mechanisms of viral pathogenesis at the molecular level is necessary to design effective and specific antiviral strategies. Much of our knowledge of viral pathogenesis is based on animal models because such systems can ...

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