A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. However, the term â€œvirusâ€ is commonly used, albeit erroneously, to refer to many different types of bad programs.
The original virus 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 the Internet, or by carrying it on a removable medium such as a floppy disk, CD, or USB drive.
Also, 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. Todayâ€™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.
The Creeper virus was first detected on ARPANET, the forerunner of the Internet in the early 1970s.
It propagated via the TENEX operating system and could make use of any connected modem to dial out to remote computers and infect them. It would display the message â€œIâ€™M THE CREEPER : CATCH ME IF YOU CAN.â€. It is rumored that the Reaper program, which 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 â€œElk Clonerâ€ is commonly credited with being the first computer virus to appear â€œin the wildâ€ â€” 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 â€œin the homeâ€.
Written in 1982 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread by floppy disk.
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.