Malicious software (Malware)

Malware is a broad category of software designed to infiltrate or damage a computer system. Types of malware include spyware, adware, Trojan horses, worms, and true viruses.

A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The original may modify the copies or the copies may modify themselves. A virus can spread from one computer to another when its host (as a floppy disk, CD, or USB drive) is taken to the uninfected computer. Viruses can also spread to other computers by infecting files on a network file system or a file system that is accessed by another computer.

Viruses are sometimes confused with computer worms and Trojan horses. A worm, however, can spread itself to other computers as part of a host. A Trojan horse is a file that is harmless until executed. In contrast to viruses, Trojan horses do not insert their code into other computer files.

Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others simply replicate themselves and perhaps make their presence known by presenting text, video, or audio message. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bugridden, and these bugs may lead to system crashes and data loss.

Many personal computers are now connected to the Internet and to local-area networks. There are many viruses operating in the Internet today, and new ones are discovered every day. The first PC virus was a boot sector virus called Brain, created in 1986 by two brothers from Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written.

Since the mid-1990s, macro viruses have become common. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel. These viruses spread in Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most of these viruses were able to spread on Macintosh computers as well. Most of these viruses did not have the ability to send infected e-mail. Those viruses which did spread through e-mail took advantage of the Microsoft Outlook COM interface.

A virus may also send a web address link as an instant message to all the contacts on an infected machine. If the recipient, thinking the link is from a friend follows the link to the website, the virus at the site can infect this new computer and continue propagating.

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