University of Wisconsin Knowledgebase

Virus - Virus Information

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This document gives an overview of computer viruses and has definitions for macro viruses, worms, and trojan horses.

What is a virus?

A computer virus is piece of computer code, usually hidden within a seemingly innocuous program, that replicates itself and inserts those copies into other programs or files. Viruses usually perform one or more malicious actions on the infected system (such as destroying data). Computer viruses never occur "naturally"; they are always man-made. Once created and released, however, their spread is not usually under direct human control.

Macro viruses

A macro is a piece of code that can be embedded in a data file. Some word processors (e.g., Microsoft Word) and spreadsheet programs (e.g., Microsoft Excel) allow you to attach macros to the documents they create. In this way, documents can control and customize the behavior of the programs that created them, or even extend the capabilities of that program. For example, a macro attached to a Microsoft Word document might be executed every time the document is saved and cause the text of the document to be run through an external spell checking program.

A macro virus is a malicious macro that is attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to document files rather than executable programs.

Worms

Worms are very similar to viruses: they are computer programs that replicate themselves, and they often (but not always) interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not embed themselves in other files or programs. Although worms are technically distinct from viruses, they are still commonly referred to as viruses.

What is a Trojan horse?

A Trojan Horse is a program that comes with a hidden surprise intended by the programmer, but totally unexpected by the user. Trojan Horses are often designed to cause damage or do something malicious to a system, but are usually disguised as something useful. Unlike viruses, Trojan Horses don't make copies of themselves. Like viruses, they can cause significant damage to a computer. Although Trojan Horses are technically distinct from viruses, they, like worms, are still commonly referred to as viruses.

Information adapted from Indiana University Knowledge Base and Symantec AntiVirus Research Center (SARC).