Viruses - Types Of Viruses

File infectors target data and executable files on a system's hard drive, and spread primarily by attaching themselves to such files as they spread through a system that uses shared programs. Since executable files are far less likely to be shared over diskettes or e-mail than are data files, according to Security Management, file infectors don't tend to be as successful at spreading outside of local networks.

Boot-sector infectors (BSIs) attack the master boot record the computer taps to start up. BSIs are among the most difficult virus programs to write, according to Security Management, and their claim on the virus population was declining since they don't tend to proliferate over networks and are spread primarily by diskette rather than by e-mail. Modern networking technology, then, was phasing out BSIs, although by the early 2000s they still constituted a significant threat.

Macro viruses attach themselves to those programs that alleviate computer users from performing repetitive tasks, and have grown more prominent in an environment of sophisticated personal computers with many automated macros. Macros are also frequently attached to data files, thus speeding the spread of macro viruses.

Worms were a particular subset of viruses that distinguish themselves by replicating across networks without ever directly attaching themselves to a host program, although the most widely publicized worm viruses were spread as e-mail attachments. Generally, worms invade an individual's computer through e-mail, and then use that individual's e-mail address list to send themselves to others. Worms are characterized by the speed with which they spread through systems; several major worms, such as Melissa and LoveLetter, spread globally before anti-virus players had even detected the problem, much less devised a disinfection program.

Trojan horses, as the Homeric name implies, distinguish themselves by deceit. They appear to the user as benign or beneficial, but instead—or in addition—perform unwanted and potentially destructive functions. Some Trojan horses directly attack files or programs, while others compromise security measures, most commonly by stealing passwords. Still other Trojan horses do no damage at all, but pretend to. These are joke or hoax programs that deceive the user into believing an infection has occurred.