Introduction :

A 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 computer viruses 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.

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 programs periodically check your computer system for the best-known types of viruses.

Some virus may display only a message on the screen. others may slow down your PC.

They can also erase files or format your floppy or hard disk and crash the system

Types of Computer Virus:

There are different types of viruses which can be classified according to their origin, techniques, types of files they infect, where they hide, the kind of damage they cause, the type of operating system, or platform they attack.

Boot Virus

The boot virus copies the virus code in the boot sector.

A virus written into the boot sectors of a floppy disk. A popular way to spread a virus when floppy disks were widely used, the boot virus relied on people forgetting to remove the last floppy they inserted when they turned the machine off.

When turned back on, the machine read the boot sector program, which normally loads the operating system, but ran the infected program instead.

Once infected, the boot virus replicated itself onto all subsequent floppies used in the machine.

Polymorphic Virus

A polymorphic virus creates an infection in a computer that is known as a polymorphic infection that creates copies of itself, with each copy different to fool a virus detection and users.

The variations are typically different forms of encryption or other signatures to make it more difficult for a virus detection program to find and remove the virus from a computer.

Because one polymorphic virus could have hundreds or thousands of variants it makes it more difficult to detect every variant of the virus.

Developers that design the detection programs have to write extra lines of code to make the programs better at detecting the virus infections.

Even the best antivirus programs can have trouble with detecting and cleaning polymorphic infections or not giving false positives.

Macro Virus

Macro virus is a computer virus that spreads to other computers through software programs that utilize macros.

For example, Microsoft Word and Microsoft Excel are two popular and widely used programs that are capable of executing macros.

Macro viruses written for these programs can spread by infecting other related documents each time the document is open.

Because these files are commonly used and sent through e-mail, a computer network can be quickly infected by these viruses.


Worms are programs that replicate and spread to other system. Often it uses a computer network to spread itself

A computer worm is a type of virus that replicates itself, but does not alter any files on your machine.

Worms are hard to detect because they are typically invisible files. They often go unnoticed until your computer begins to slow down or starts having other problems.

Unlike Trojan horses, worms can replicate themselves and travel between systems without any action from the user.

For these reasons, it is good to have an anti-virus program installed on your system that can detect and remove worms before they have a chance to replicate or spread to other computers.


A trojan is a program file that claims to do something. but it actually does something that is not required. Trojans are also known to create back doors to give malicious users access to the system.

It is a harmful piece of software that looks legitimate. Users are typically tricked into loading and executing it on their systems.

After it is activated, it can achieve any number of attacks on the host, from irritating the user (popping up windows or changing desktops) to damaging the host (deleting files, stealing data, or activating and spreading other malware, such as viruses).

How to detect Virus from Computer:

Enter Safe Mode.Keep your PC disconnected from the Internet, and don't use it until you're ready to clean your PC. This can help prevent the virus from spreading and/or leaking your private data.

Delete Temporary Files. Doing this may speed up the virus scanning, free up disk space, and even get rid of some virus.

Download Malware Scanners. If you already had an anti-virus program active on your computer, you should use a different scanner for this malware check, since your current anti-virus software may have not detected the malware.

Once infected, the boot virus replicated itself onto all subsequent floppies used in the machine

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