What are viruses, worms, and Trojan horses?

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Defining viruses, worms, and Trojan horses

A computer virus, according to Webster's Collegiate Dictionary, is "a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs or files, and that usually performs a malicious action (such as destroying data)". Two categories of viruses, macro viruses and worms, are especially common today. Computer viruses are never naturally occurring; they are always man-made. Once created and released, however, their spread is not directly under human control.

Macro viruses: A macro is a piece of code that can be embedded in a data file. A macro virus is a virus that exists as a macro attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to data files (i.e., documents) rather than executable programs. If you are unable to save a document in Microsoft Word, your computer may have a macro virus; see What is Microsoft Word's macro virus protection tool, and how do I activate it?
Document-based viruses are, and will likely continue to be, more prevalent than any other type of virus.
Worms: Worms are very similar to viruses in that they are computer programs that replicate functional copies of themselves (usually to other computer systems via network connections) and often, but not always, contain some functionality that will 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 attach themselves to other files or programs. Because of their similarity to viruses, worms are often also referred to as viruses.
Trojan horses: A Trojan horse is a program that does something undocumented which the programmer intended, but that the user would not approve of if he or she knew about it. According to some people, a virus is a particular case of a Trojan horse, namely one which is able to spread to other programs (i.e., it turns them into Trojans too). According to others, a virus that does not do any deliberate damage (other than merely replicating) is not a Trojan. Finally, despite the definitions, many people use the term "Trojan" to refer only to a non-replicating malicious program.

Resources and more information about viruses

For details on avoiding viruses, see Best practices for computer security.
For virus information of particular interest to Indiana University, visit Protect IU, the University Information Security Office's (UISO) web page.
For news about current viruses, check Symantec's Security Response site, the McAfee Security Center, and the Computer Associates Virus Encyclopedia.
For news about the antivirus industry, see the SecurityFocus virus page.

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Last modified on August 26, 2011.