Computer virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves. All computerviruses are man-made. A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems.
The first academic work on the theory of computer viruses (although the term "computer virus" was not used at that time) was done in 1949 by John von Neumann who gave lectures at the University of Illinois about the "Theory and Organization of Complicated Automata". The work of von Neumann was later published as the "Theory of self-reproducing automata". In his essay von Neumann described how a computer program could be designed to reproduce itself.
In 1972 Veith Risak, directly building on von Neumann's work on self-replication, published his article "Selbstreproduzierende Automaten mit minimaler Informationsübertragung" (Self-reproducing automata with minimal information exchange). The article describes a fully functional virus written in assembler language for a SIEMENS 4004/35 computer system.
An article that describes "useful virus functionalities" was published by J. B. Gunn under the title "Use of virus functions to provide a virtual APL interpreter under user control" in 1984.
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The Terminal Man, a science fiction novel by Michael Crichton(1972), told (as a sideline story) of a computer with telephone modem dialing capability, which had been programmed to randomly dial phone numbers until it hit a modem that is answered by another computer. It then attempted to program the answering computer with its own program, so that the second computer would also begin dialing random numbers, in search of yet another computer to program. The program is assumed to spread exponentially through susceptible computers.
The Creeper virus was first detected on ARPANET, the forerunner of the Internet, in the early 1970s. Creeper was an experimental self-replicating program written by Bob Thomas at BBN Technologies in 1971. Creeper used the ARPANET to infect DEC PDP-10 computers running the TENEX operating system. Creeper gained access via the ARPANET and copied itself to the remote system where the message, "I'm the creeper, catch me if you can!" was displayed. The Reaper program was created to delete Creeper.
Since 1987, when a virus infected ARPANET, a large network used by the Defense Department and many universities, many antivirus programs have become available. These programsperiodically check your computer system for the best-known types of viruses.
Some people distinguish between general viruses and worms. A worm is special type of virus that can replicate itself and use memory, but cannot attach itself to other programs.
Academic work
The first academic work on the theory of computer viruses (although the term "computer virus" was not used at that time) was done in 1949 by John von Neumann who gave lectures at the University of Illinois about the "Theory and Organization of Complicated Automata". The work of von Neumann was later published as the "Theory of self-reproducing automata". In his essay von Neumann described how a computer program could be designed to reproduce itself.In 1972 Veith Risak, directly building on von Neumann's work on self-replication, published his article "Selbstreproduzierende Automaten mit minimaler Informationsübertragung" (Self-reproducing automata with minimal information exchange). The article describes a fully functional virus written in assembler language for a SIEMENS 4004/35 computer system.
In 1980 Jürgen Kraus wrote his diplom thesis "Selbstreproduktion bei Programmen" (Self-reproduction of programs) at the University of Dortmund. In his work Kraus postulated that computer programs can behave in a way similar to biological viruses.
In 1984 Fred Cohen from the University of Southern California wrote his paper "Computer Viruses - Theory and Experiments".It was the first paper to explicitly call a self-reproducing program a "virus", a term introduced by Cohen's mentor Leonard Adleman. In 1987, Fred Cohen published a demonstration that there is no algorithm that can perfectly detect all possible viruses.
Science fiction
The actual term "virus" was first used to denote a self-reproducing program in a short story by David Gerrold in Galaxy magazine in 1969—and later in his 1972 novel, When HARLIE Was One. In that novel, a sentient computer named HARLIE writes viral software to retrieve damaging personal information from other computers to blackmail the man who wants to turn him off.
Virus programs
A program called "Elk Cloner" was the first personal computer virus to appear "in the wild"—that is, outside the single computer or lab where it was created.Written in 1981 by Richard Skrenta, it attached itself to the Apple DOS 3.3 operating system and spread via floppy disk. This virus, created as a practical joke when Skrenta was still in high school, was injected in a game on a floppy disk. On its 50th use the Elk Cloner virus would be activated, infecting the personal computer and displaying a short poem beginning "Elk Cloner: The program with a personality."
The first IBM PC virus in the wild was a boot sector virus dubbed (c)Brain, created in 1986 by the Farooq Alvi Brothers in Lahore, Pakistan, reportedly to deter piracy of the software they had written.
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, usually inadvertently. Personal computers of the era would attempt to boot first from a floppy if one had been left in the drive. Until floppy disks fell out of use, this was the most successful infection strategy and boot sector viruses were the most common in the wild for many years.
Traditional computer viruses emerged in the 1980s, driven by the spread of personal computers and the resultant increase in BBS, 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 BBSs.[citation needed] Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by other computers.
Macro viruses have become common since the mid-1990s. Most of these viruses are written in the scripting languages for Microsoft programs such as Word and Excel and spread throughout Microsoft Office by infecting documents and spreadsheets. Since Word and Excel were also available for Mac OS, most could also spread to Macintosh computers. Although most of these viruses did not have the ability to send infected email messages, those viruses which did take advantage of the Microsoft Outlook COM interface.
Some old versions of Microsoft Word allow macros to replicate themselves with additional blank lines. 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.
Viruses that spread using cross-site scripting were first reported in 2002,and were academically demonstrated in 2005.There have been multiple instances of the cross-site scripting viruses in the wild, exploiting websites such as MySpace and Yahoo!.
How to remove and avoid computer viruses
A computer virus is malicious software (also known as "malware") that can copy itself and infect other software or files on your computer.
If you suspect your computer has been infected, the Microsoft Windows website provides step-by-step instructions for removing viruses and other malware.
Fortunately, if you update your computer and use free antivirus software such as Microsoft Security Essentials, you can help permanently remove unwanted software and prevent installation in the first place.
Note What you might think is a virus could actually be spyware. Microsoft Security Essentials helps protect against spyware, too.
How do I know if my computer has been infected?
After you open and run an infected program or attachment on your computer, you might not realize that you've introduced a virus until you notice something isn't quite right.
Here are a few indicators that your computer might be infected:
- Your computer runs more slowly than normal
- Your computer stops responding or freezes often
- Your computer crashes and restarts every few minutes
- Your computer restarts on its own and then fails to run normally
- Applications on your computer don't work correctly
- Disks or disk drives are inaccessible
- You can't print correctly
- You see unusual error messages
- You see distorted menus and dialog boxes
These are common signs of infection—but they might also indicate hardware or software problems that have nothing to do with a virus. Unless you run the Microsoft Malicious Software Removal Tool and install industry-standard, up-to-date antivirus software on your computer, there is no way to be certain if your computer is infected with a virus or not.
Tip Beware of messages warning you that you've sent email that contained a virus. This can indicate that the virus has listed your email address as the sender of tainted email. This does not necessarily mean you have a virus. Some viruses have the ability to forge email addresses. In addition, there is a category of malware called rogue security software that works by causing fake virus alerts to pop up on your computer.
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