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A program called Elk Cloner is credited with being the first computer virus to appear in the wild -- that is, outside the single computer or lab where it was created. Written in 1982 by Rich Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk.
The first PC virus was a boot sector virus called (c) Brain, created in 1986 by two brothers, Basit and Amjad Farooq Alvi, operating out of Lahore, Pakistan. The brothers reportedly created the virus to deter pirated copies of software they had written. However, analysts have claimed that the Ashar virus, a variant of Brain, possibly predated it based on code within the virus.
Before computer networks became widespread, most viruses spread on removable media, particularly floppy disks. In the early days of personal computers, many users regularly exchanged information and programs on floppies. Some viruses spread by infecting programs stored on these disks, while others installed themselves into the disk boot sector, ensuring that they would be run when the user booted the computer from the disk.
Traditional computer viruses were mostly first seen at the last half of the 1980s, and they came about because of a few reasons. The first reason was the spread of personal computers. Prior to the 1980s, home computers were nearly non-existent or they were toys. Real computers were rare, and experts locked them away for use. During the 1980s, real computers started to spread to businesses and homes because of popularity. By the late 1980s, PCs were widespread in businesses, homes, and college campuses.
The second reason was the use of bulletin boards on the computer. People could dial up a bulletin board with a modem and download all sorts of different programs. Most popular were games, and then simple word processors, spreadsheets, etc. Bulletin boards led to what is now known as the virus called a Trojan horse. The third reason that led to the creation of viruses was most definitely the floppy disk. At the end of the 1980s, programs were very small, and could fit the operating system, a word processor, and many documents onto a single floppy disk. Most computers did not have hard disks, so you would turn on your machine and it would load the operating system and everything else straight from the floppy disk. Viruses took advantage of these three facts to create the first self-replicating programs.
As bulletin board systems and online software exchange became popular in the late 1980s and early 1990s, more viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBSes. Within the pirate scene of hobbyists trading illicit copies of commercial software, traders in a hurry to obtain the latest applications and games were easy targets for viruses.
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. Numerically, most of these viruses did not have the ability to send infected e-mail. The ones that did usually worked by accessing the Microsoft Outlook COM interface.
Macro viruses pose unique problems for detection software. For example, some versions of Microsoft Word caused macros to replicate themselves with additional blank lines. The virus behaved identically but would be misidentified as a new virus. In another example, if two macro viruses simultaneously infect a document, the combination of the two, if also self-replicating, can appear as a mating of the two and would likely be detected as a virus unique from the parents.
A computer virus may also be transmitted through instant messaging. A virus may 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 (a trusted source) and follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.