As long as computers have been stable enough to be useful, people have managed to find ways to cause problems and one of the most effective ways to create mischief on computers is through malware like a computer worm or virus, just like their biological analog. These types of programs will infect a computer before self-replicating by spreading themselves to other machines. Now, whatever prank was intended to be pulled can be run on hundreds or thousands or possibly even millions of computers all because the program itself was designed to travel. Well, the idea of an autonomously self-replicating entity goes back to the 1940s. It wouldn’t be until the early 70s that such a program would exist.

In 1967 the app named creeper was released onto the ARPANET where it would bounce around between the computers, the only trace it left was a message printed out onto the teletype terminal, reading-“I’M THE CREEPER: CATCH ME IF YOU CAN!!”. Later versions of the program would drop a copy of itself on the visiting machine, making creeper the first program to automatically spread copies of itself to other machines. In other words FIRST COMPUTER WORM. Eventually, the worms creator Ray Tomlinson tired of the nuisances program caused created the world’s second computer worm REAPER, designed to clean up the mess that the creeper had created across the network.

Just a few years later the first worm to appear in the wild would emerge. Though like the creeper, ANIMAL was never designed with no intentions. Since it was written in an era of text-based games, ANIMAL was a popular program among Univac users, where the computer would try to guess what animal the user was thinking of through a series of yes-or-no questions. After improving the game with the ability to learn from users and expand its database of animals, John Walker received a ton of requests for the program. Which he fulfilled through a new way of distributing software.

John Walker added a routine to the program called PURVEYED, which in the background as the user is playing would make a copy of the animal to every folder the user had access to. In the case of a superuser, the program would be able to copy itself to every directory in the system, and since tape sharing was a common just between users of the time, Animal found its way on the system where it had never even been requested in the first place.

One of the main reasons why these worms were so effective was the fact that they were set loose on multi-user systems. Meaning that a worm could either propagate by way of a network like with creeper, worth or resources shared between users in the case of animals. But we cannot say that personal computers were entirely immune to these types of programs. All it took was a small tweak in the mechanism they used to spread.

Richard Skrenta an Apple 2 enthusiast in 1982 at the time networking between personal computers was still a rarity, the primary medium through which shareware was distributed was “SneakerNet”. Skrenta had developed a reputation for enhancing the discs he shared. His efforts eventually resulted in the world’s first Computer Virus. Unlike worms, a virus spreads by inserting itself into the other program.

In the case of Skrenta’s program, the virus would add itself to the boot sector of the system discs used to boot the apple too, which in turn would then add the cloner to every other disc loaded into the system. By writing into the boot sector, not only would the cloner run every time that disk was loaded, but it was also able to spread almost invisibly, only showing itself on every 50th boot. This is how the viruses kept on spreading from then on.


VIRUS stands for Vital Information Resource Under Seize. A computer virus is a malicious program or software which is loaded onto a user’s computer without the user’s knowledge. Computer virus also performs malicious actions and tries to access files and folders, and do stuff which requires special permission.

The term ‘Computer Virus’ was first formally defined by Fred Cohen in 1983.
Computer Viruses never occur naturally. Meaning, people code and create computer viruses and then release them. However, their diffusion is not directly under human control. After entering, a computer virus attaches itself to another program in such a way that the execution of the host program triggers the action of the computer virus.

For example, a computer virus might attach to an excel file and whenever the user executed or opens the Excel File, the virus will start executing the host trigger action. The trigger action is malicious and it can be something that duplicates and replicates the virus itself. It can self replicate and insert itself onto other programs and files.
For example, if you have an infected Excel file, opening it will spread the virus to other excel files and other types of files usually.

Besides replicating, a computer virus also has a program that destroys data. Some viruses start working as soon as their code is executed. While others, lie dormant until a particular event (as programmed) gets initiated. For example, a virus will start deleting or destroying data and copying data, once it is executed. Whereas other viruses will lie down and wait for a trigger.


Viruses spread when the software or documents that they are attached to are transferred from one computer to another. For example, if your friend has an infected MP4 file or a movie, transferring it to your computer will infect your computer. Some can infect files without increasing their file size while invading detection by killing the task associated with anti-virus.

Let us take another example, an Excel file’s size will be increased if something is attached to it, or if some more data or more information is added to the file. Some viruses will avoid increasing the size and another method of avoiding detection is by killing the anti-virus itself.

Many viruses come with a program that allows you to kill modern anti-virus software. Usually, viruses are created to work with popular anti-viruses. For example, since Microsoft’s virus removal tool is quite popular and even Avast is a leading anti-virus, most viruses will be programmed to kill these anti-viruses.


It fixes itself into the system’s memory and gets activated when the OS runs and infects all the files that are then opened.
It hides in the RAM and stays there even after the malicious code is executed.
E.g: Meve, Randex, etc.

It comes into action when a file containing the virus is executed. It replicates and infects files in the folders straight away. The main target of this virus are .exe type and .com files.

E.g.: Vienna Virus.

It deletes the information contained in the files that it infects, rendering them partially or useless once they have been infected.
E.g.: Way, Trj.Reboot.

Also called as Master boot sector virus or Master boot record virus.
It affects the boot sector of a hard disk.
E.g.: Polyboot. B

It infects files that are created using certain applications or programs that contain macros like .doc, .xls, .ppt
E.g.: Melissa.

It is used to store all the information about the location of files in unusable space.
E.g.: Linkvirus.