Viruses have been around for more than 15 years. According to Symantec's Antivirus Research Center, there was one known virus in 1986; six in 1989 and 80 by 1990. Today, there are over 42,000 known virus and worm threats in existence.

Viruses usually work by infecting a file - such as a document, spreadsheet or picture file - and then work stealthily in the background. Sometimes viruses lie dormant for a long time until "activated" by a user function such as opening a program or attachment. Other viruses launch themselves immediately upon entering the target computer and can do their damage almost instantly.

Traditionally, viruses spread by diskette; someone would give a file to another person without knowing that a virus was on the disk, and the virus would spread slowly as the file was shared between co-workers, departments and companies. Today, viruses spread far more commonly via email and networks as diskettes become less useful with today's larger file sizes. As more and more people get always-on internet connections such as DSL, the threat of contracting a virus becomes a 24-hour possibility.

Why do viruses exist?
Viruses are created by different people for different reasons. Humorous harmless viruses are created by people who want to "spread their joke" to a wide body of people. Sometimes, these types of viruses are just "hoaxes" in that they cause more panic than real harm to people's computers. On the other hand, the more destructive viruses are created by unscrupulous individuals who act as common criminals. Like someone who enters your office and simply takes a sledgehammer to your computer, such people create viruses for a variety of reasons, usually none of them good. It has even been suggested that viruses are purposely created as forms of "corporate or government espionage" because they can target competitors or unfriendly nations and try to bring their computing systems to a halt. Whatever the reasons viruses exist, their nature is evidence enough that they are malicious.