A computer virus is generally defined as a program that modifies other programs to contain a (possibly altered) version of itself, an approximation to a mathematical definition given by Dr. Frederick Cohen in his doctoral thesis. In other words, a virus requires a host to attach itself to, either by modifying the host, or inserting itself in some way into the “chain of command.” In this way, the virus not only ensures that it’s executed whenever its host is launched, but is also able to self-replicate, as infected files are copied, downloaded or otherwise transferred to other computer systems. Its name is based on the fact that its behavior is very roughly analogous to that of biological viruses.

There are several categories of computer viruses, depending on what type of files they infect and how.

See also: Boot sector virus, File Virus,

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