The Creeper virus was first detected on ARPANET, the forerunner of the Internet in the early 1970s. It propagated via the Tanex operating system and could make use of any connect modem to dial out to remote computers and infect them. It would display the message "I'M THE CREEPER : CATCH ME IF YOU CAN.". It is rumored that the Reaper program that appeared shortly after and sought out copies of the Creeper and deleted them, may have been written by the creator of the Creeper in a fit of regret.
A program called "Elk Cloner" is commonly credited with being the first computer virus to appear "in the wild" — that is, outside the single computer or lab where it was created, but that claim is false. See the Timeline of notable computer viruses and worms for other earlier viruses. It was however the first virus to infect computers "in the home". Written in 1982 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk.[1] This virus was originally a joke, created by a high school student and put onto a game. The disk could only be used 49 times. The game was set to play, but release the virus on the 50th time of starting the game. Only this time, instead of playing the game, it would change to a blank screen that read a poem about the virus named Elk Cloner. The poem that showed up on the screen is as follows: It will get on all your disks. It will infiltrate your chips. Yes it's Cloner! It will stick to you like glue. It will modify RAM too. Send in the Cloner! The computer would then be infected.

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 the personal computer, 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 emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS and modem use, and software sharing. Bulletin board driven software sharing contributed directly to the spread of Trojan horse programs, and viruses were written to infect popularly traded software. Shareware and bootleg software were equally common vectors for viruses on BBS's. Within the "pirate scene" of hobbyists trading illicit copies of retail 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. 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.Macro viruses pose unique problems for detection software. For example, some versions of Microsoft Word allowed 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 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 (a trusted source) follows the link to the website, the virus hosted at the site may be able to infect this new computer and continue propagating.The newest species of the virus family is the cross-site scripting virus. The virus emerged from research and was academically demonstrated in 2005 [3]. This virus utilizes cross-site scripting vulnerabilities to propagate. Since 2005 there have been multiple instances of the cross-site scripting viruses in the wild, most notable sites affected have been MySpace and Yahoo.

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, as occurs in a metamorphic virus. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable medium such as a floppy disk, CD, USB drive or by the Internet. Additionally, viruses can 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 can spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a file that appears harmless until executed.Many personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware.Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. 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 bug-ridden, and these bugs may lead to system crashes and data loss.