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In addition to the effects on the lungs, many viruses affect other organs and can lead to illness affecting many different bodily functions. Some viruses also make the body more susceptible to bacterial infection; for this reason, bacterial pneumonia often complicates viral pneumonia.
This can be measured as virulence, which can be used to compare the quantitative degree of pathology between related viruses. In other words, different virus strains possessing different virus factors can lead to different degrees of virulence, which in turn can be exploited to study the differences in pathogenesis of viral variants with ...
Viruses, however, use a completely different mechanism to cause disease. Upon entry into the host, they can do one of two things. Many times, viral pathogens enter the lytic cycle; this is when the virus inserts its DNA or RNA into the host cell, replicates, and eventually causes the cell to lyse, releasing more viruses into the environment.
Viruses can reproduce only inside cells, and they can gain entry by using many of the receptors involved in immunity. Once inside the cell, viruses use the cell's biological machinery to their own advantage, forcing the cell to make hundreds of identical copies of themselves.
Some viruses can "hide" within a cell, which may mean that they evade the host cell defenses or immune system and may increase the long-term "success" of the virus. This hiding is deemed latency. During this time, the virus does not produce any progeny, it remains inactive until external stimuli—such as light or stress—prompts it to activate.
The virus can reactivate and begin producing large amounts of viral progeny (the lytic part of the viral life cycle) without the host becoming reinfected by new outside virus, and stays within the host indefinitely. [2] Virus latency is not to be confused with clinical latency during the incubation period when a virus is not dormant.
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Viruses enter host cells using a variety of mechanisms, including the endocytic and non-endocytic routes. [4] They can also fuse at the plasma membrane and can spread within the host via fusion or cell-cell fusion. [5] Viruses attach to proteins on the host cell surface known as cellular receptors or attachment factors to aid entry. [6]