A virus is a small piece of software that piggybacks on a program you use or on certain files that automatically open a program you use. Computer viruses can make copies of themselves and can spread from computer to computer when files are shared, usually over a network, sometimes as attachements to e-mail messages. Some viruses are harmless but others have the potential to create havoc on the systems they infect.

What types of viruses are there?

• Program viruses infect computer programs and become active when the infected program is run.
• Boot sector viruses infect diskettes and hard disks and become active when an infected disk is used to start the computer.
• Macro viruses infect documents (files) through the macro programming capabilities of some programs. Macro viruses become active when an infected document is opened, and the program opening the document has its macro capabilities turned on (enabled).
• E-mail viruses can infect your desktop email system. They spread through e-mail messages and usually replicate by automatically e-mailing copies to all entries in your e-mail address book.
• Worms are programs that are able to replicate on their own over computer networks. A worm, technically, differs from a computer virus in how it spreads: that is, a virus attaches itself to another program or file and spreads as its "host" comes in contact with other computers, whereas a worm scans a network for computers with same security hole that it can exploit to infect that machine, scan other machines for that security loophole, and continue replicating itself independently.
• Trojan horses are programs that claim to do one thing but really do something else (usally malicious). Since they have no way to replicate automatically, they, like worms, are not techically computer viruses. But they still can damage your computer, launch a virus, or launch a worm.