Computer Viruses, what they are and what they do

A computer virus is a self-replicating program that explicitly copies itself and that can infect other programs by modifying them or their environment such that a call to an infected program implies a call to a possibly evolved copy of the virus. Note that 'program' takes a fairly liberal interpretation here, involving much more than the 'obvious' application programs (executables) in a typical computer system. Almost any code that is executed or interpreted may be 'virusable' so long as, when running in its normal execution context, that code has write access to some other executable object (note this need not be the same kind of executable object!).

Another important feature of viruses is that, unlike their biological namesakes, they need not be parasitic. Various companion infection methods exist and mechanisms that involve altering the behavior of the host program's environment, rather than altering the program itself, can be sufficient to classify a program as viral (so long as it is also self-replicating).

Worms are, in some ways, similar to viruses in that they make copies of themselves. However, there is a deal of disagreement between researchers over how to classify worms. See the worm entry for more discussion of this issue.

When discussing viruses, it is common to hear talk about obvious symptoms and damaging payloads. Some viruses display symptoms, and some cause damage to files in a system they have infected, but neither symptoms nor damage are essential in the definition of a virus. A non-damaging virus is still a virus, not a prank.

Many viruses cause intentional damage. But many more cause damage that may not have been intended by the virus' writer. For instance, when a virus finds itself in a very different environment from that for which it was written, a non-destructive virus can suddenly become very destructive. A good case in point are many common (or formerly common) boot viruses: while a particular boot virus might not contain any code to damage computers running Windows NT, booting an NT machine with such a virus is likely to result in system repairs the user or system administrator may not have been prepared for.