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What Is Computer Virus And Types Of Virus

What is Computer virus

A computer virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Computer Virus 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.This article contains ,what is computer virus,working of computer virus,types of computer virus.

Computer Virus
Computer Virus

Different types of computer virus act in different ways depending on what the author intends to achieve. Most viruses fall under three main categories all of which we will go over briefly to gain a better understanding of how they work. 

Types Of Computer Virus

Program viruses

These types of computer virus will infect only executable files (with extensions like .BIN, .COM, .EXE, .OVL, .DRV, and .SYS). Once executed, these programs load into memory, along with the virus contained within them. Once in the memory, the scenario repeats – the virus is free to act and infect other files or simply deliver its payload. These viruses are friendlier than boot viruses and can be removed a lot easier.

Example: Sunday, Cascade

Multipartite viruses

These computer virus are hybrids, derived from boot viruses and program viruses. They infect executable files, just like the program viruses but, once the executable is executed, it infects the master boot records. The scenario is similar to the boot virus’s one: once you boot your operating system, the virus is loaded into the memory, from where it is free to infect other programs and replicate itself, ultimately delivering the payload.

Examples : Invader, Flip, and Tequila

Stealth viruses

These computer virus are specialized in avoiding detection and will use a number of techniques to do so. Most of them simply redirect the hard disk head, forcing it to read another memory sector instead of their own. Some of them also alter the reading of the file size shown when listing the directory. These types of computer virus are very hard to find by humans, but antivirus software is specially designed to track them down and erase them.

Examples:  Frodo, Joshi, Whale

Boot viruses

Since nobody uses floppy disks anymore, these types of computer virus are left to infect only the master boot records of the hard disk. The boot record program loads the operating system in the memory at startup. These viruses replace the boot record and move it to a different part of the hard disk, or simply overwrite it. As you can imagine, once you start your operating system, it loads into memory, along with the virus. Once the virus is in the memory, it is free to do whatever it was programmed to do. To avoid loading the boot record viruses into memory, you must boot your operating system from another source (another hard drive or a bootable CD/DVD).

Examples: Form, Disk Killer, Michelangelo, and Stone virus 

Active X viruses

Most people do not know how to configure ActiveX and Java controls, unconsciously leaving a security hole. Applets are then allowed to run freely on the machine, delivering all ActiveX viruses. By simply turning off some ActiveX and Java controls in the browser, a user can efficiently protect their PC from this type of computer virus.

Macro viruses

This virus is relatively new and it infects macros within a template or document. When you open a word processing document, it activates the virus. The virus infects the Normal.dot template, which is a general file used by all the documents. So, whenever you open an uninfected document, by referring to the Normal.dot file, it gets infected as well. This infection can only spread if infected documents are opened on another machine.

Polymorphic viruses

The polymorphic viruses will always change their source code from one infection to another. Each infection is different, and this makes detection very hard. However, detection is still possible, depending on the antivirus.