Software Quarterly

A Virus by Any Other Name


A virus is a computer program that alters, or "infects," other programs with a copy of itself, and causes the altered program to behave differently.

Effects of computer viruses can range from an unexpected message displayed across a computer screen ... to rearranged colors or letters in a file ... and to complete destruction of all data on a hard disk. But many viruses will have no readily apparent, visible effect; users may not know their systems are infected until it's too late. One of the characteristics of viruses that makes them extremely dangerous is their ability to spread quickly and easily -- from program to program and computer to computer.

Viruses usually penetrate a system from a diskette, although it's sometimes an electronic connection to a network or a bulletin board service that lets the virus in. Once inside, viruses hide in a computer's memory, boot sectors, and executable files. Often, viruses replicate and attach themselves to other executable files every time the user runs the program to which they are attached. Eventually, when the user performs a certain action, or a computer condition occurs, or a particular date is reached -- the virus is activated.

To make the situation even worse, computer viruses, like those that infect humans, come in many forms. Experts estimate that two to three new viruses are created every day, and that between 3,000 and 6,000 known viruses exist today, depending on whether different strains of the same virus are counted.

Many new viruses exhibit characteristics common among viruses and so are easily detected. However, some viruses are written with sophisticated techniques that make their detection more difficult.

For example, by hiding certain identifier codes that are present in most viruses, a "stealth" virus can trick an operating system into treating infected files as if they were normal. Polymorphic viruses elude capture by mutating themselves into an almost completely different form every time they replicate. The process is so random that the next form can't be predicted readily by anti-virus software.

Related to viruses are virus-like programs, which can't replicate themselves but can cause effects similar to those of regular viruses. These programs may be "logic bombs," which wait for the user to take a specific action; "time bombs," which activate on a particular date; "Trojan horses," which appear harmless until the infected program is opened; or "worms," which burrow through or between networks, changing or overwriting data as they go.


See also:
How Much is Enough?
Stealth Bomber: The Computer Virus Writer
Common Viruses
And Still More Viruses

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