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PATHOGENESIS OF VIRAL INFECTION

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Most cells and organisms do not passively submit to virus infection. As noted in Chapter 1, the response of organisms to virus infection is a major feature of evolutionary change in its most general sense. As briefly noted, a complete understanding of pathogenesis requires knowledge of the sum total of genetic features a virus encodes that allows its efficient spread between individual hosts and within the general population of hosts. Thus, the term pathogenesis can be legitimately applied to virus infections of multicellular, unicellular, and bacterial hosts.

A major challenge for viruses infecting bacteria and other unicellular organisms is finding enough cells to replicate in without isolating themselves from other populations of similar cells. In other words, they must be able to “follow” the cells to places where the cells can flourish. If susceptible cells can isolate themselves from a pathogen, it is in their best interest to do so. Conversely, the virus, even when constrained to confine all its dynamic features of existence to the replication process per se, must successfully counter this challenge or it cannot survive.

In some cases, cells can mount a defense against virus infection. Most animal cells react to infection with many viruses by inducing a family of cellular proteins termed interferons that can interact with neighboring cells and induce those cells to become wholly or partially resistant to virus infection. Similarly, some viral infections of bacterial cells can result in a bacterial restriction response that limits viral replication. Of course, if the response is completely effective, the virus cannot replicate. In this situation, one cannot study the infection, and in the extreme situation, the virus would not survive.

Viruses that infect multicellular organisms face problems attendant with their need to be introduced into an animal to generate a physiological response fostering the virus's ability to spread to another organism (i.e., they must exhibit virulence). This process can follow different routes.

Disease is a common result of the infection, but many (if not most) viral infections result in no measurable disease symptoms – indeed, inapparent infections are often hallmarks of highly coevolved virus–host interactions. But inapparent or asymptomatic infections can be seen in the interaction between normally virulent viruses and a susceptible host as a result of many factors. A partial list includes the host's genetic makeup, host health, the degree of immunity to the pathogen in the host, and the random (stochastic) nature of the infective process.

Basic Virology

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