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Everyone knows the story of the little virus that entered the cell, reproduced, and left, lysing the cell on the way. And everyone knows that this is bound to cause disease; after all, numerous cells being killed by viral particles can't be a good thing. But what a lot of people don't realize is that this is not the way viral infections always work. Disease states can be caused not only by the actions of the virus in question but also by the body's response to the virus. And, as will be the point of discussion here, viral infections do not always follow the simple pathway explained here.

The "little virus" scenario we talked about above was an example of a productive infection (one producing progengy) or a permissive cell (one the virus could replicate in) by a cytocidal virus (one that kills the cell). This is not always the case. Often viral infection may be nonproductive (also called abortive). Many viruses are noncytocidal. And many viruses are a combination of these two factors. Because of this, a great deal of study has been done in an effort to determine what shapes an infection.

The first question is, of course, what is necessary for a virus to infect a cell productively? What criteria must be met? Well first off, the virus must find a susceptible cell. A susceptible cell is one that the virus is able to infect and is often defined as the viral host. But this is not enough. The virus must also find a permissive cell, or one that is not able to stop the virus from replicating. Nonpermissive cells may be ones that are mutated such that a factor necessary to viral reproduction is not present or one detrimental to viral reproduction is present. An example of a nonpermissive cell is one with a lack of viral receptors (which prohibit attachment and hence replication). Another example may be a cell that lacks a proteolytic component necessary for viral capsid processing. Indeed, any step in the basic viral life cycle may be blocked to produce a nonpermissive cell. Finally, the third major criterion for a productive infection is the virus itself; it must not be defective for the particular host. Examples are defective interfering (DI) mutants and conditional lethal mutants. These virii need certain additional factors that may or may not enable them to replicate even in a susceptible, permissive cell.

Whether or not the infection is productive or not the infection may be lytic or nonlytic. At first this is somewhat confusing, as it is hard to imagine a virus that is unable to replicate itself doing any harm. But we'll get to that in a minute, and you shall see that it does indeed make sense. First though, consider a productive infection. There are examples of both lytic and nonlytic productive infections. In some of these so many virions are produced that the cell literally bursts, spewing forth its contents. Other times so many virions are produced and so many bud so rapidly that the cell is virtually torn apart. But in other cases, such as arenaviruses, the viruses can replicate normally without killing the cell. So we can definitely agree that a productive infection can be either cytocidal or noncytocidal. Now back to nonproductive infections. What about these? Well, to begin with, it is relatively easy to see how they may be noncytocidal; the virus may be stopped and degraded, or it may form an episome and sit harmlessly, or so forth. And indeed this is usually the case. However, there are execptions, and one of these is very important. It involves a process known as transformation.

Transformation can occur with either productive or non productive viruses. Viruses that are able to transform cells are called oncogenic viruses. When these viruses infect a cell one or more copies of the genome (in DNA form) is incorporated into the cellular genome. Here it is retained indefinitely inside the cell. This permanently alters (or transforms) the cell. Generally this is not lethal to the cellin any way. However, this can give rise to malignancy, or cancer.

The rest of this will be finished later as it is just plain taking way too long.