What is a Computer Virus and types of virus

What is a Computer Virus?

"Computer virus is a noxious program that is usually developed to create some problems in the computer system.Usually this type of computer program is imposed on the user and is intended to generate problems in the system"

The definition of computer virus helps us understand “what is a computer virus”. As the definition of program in computer states: “A computer program is used to solve different problems according to the set of instructions given to computer". It sounds from the definition of computer program that it is one of the most veracious constituents of a computer system. However the definition of computer virus is a conflict on this. The basic purpose of this computer program is to affect the working of the system adversely either by slowing down the processor's speed, deleting or corrupting important files or infecting other installed programs by making them error prone. As this superfluous program is entered into the computer system, the system might behave weirdly. Either it makes the processor slow by running as a loop repeatedly or it may spoil your important data. Sometimes you might not have any idea that your computer have been got infected by a virus as it silently do evil to your system by deleting your important files from the disk. Sometimes, people don’t have any idea that what exactly is a computer virus and how to get rid of it. People assume that their system has got out of order or there is some problem in the operating system due to which their files have got corrupted or deleted.

Now the question arises "why would anybody want to install this program into his computer system if this program is this alarming?" Well; as it has been stated before, virus is an undesirable program that is imposed on the user by an unknown person. Most commonly, computers fall a victim to virus when a user clicks an email or a link to which virus is attached.

Users can get rid of this virus problem by installing Antivirus Program in their computers. As obvious from the name, antivirus program intends to kill the virus that could be harmful for our computer. The program detects the virus by scanning the system and fights it to remove it from computer.

Some important types of computer virus and their effects are given below:

1. Resident Viruses

As it is clear from its name, the resident virus gets residence for itself in the RAM i.e. it stores itself in the main memory and then starts infecting several files even making them corrupt and error prone.

Examples of Resident virus include Randex, CMJ, Meve, and MrKlunky.

2. Multipartite Viruses

Multipartite viruses are usually spread through infected media. Just like resident virus, multipartite virus also hides in the memory. From here, it slowly moves to the boot sector of the hard drive and infects executable files on the hard drive and later across the computer system.

Example of Multipartite virus includes Ghostball .

3. Direct Action Viruses

The main purpose of this virus is replication. This virus spreads infection every time the code is executed. Under specific conditions, the virus will go into action and starts infecting files in the directory or folder that it is in.

Example includes VCL.428 .

4. Directory Virus

Directory viruses change the paths that direct towards the location of a file. It means that when we try to execute a program we are unintentionally executing the virus program which has been replaced by the corresponding program file which we wanted to execute.

Example of Directory virus includes Dir-2 .

5. Boot Virus

This type of virus attacks the boot sector of a floppy or a hard disk. The most considerable way of avoiding boot viruses is to make sure that floppy disks are write-protected and never start your computer with an unknown floppy disk in the disk drive.

Examples of Boot viruses include Polyboot.B, AntiEXE.

6. Overwrite Viruses

Files stored in the hard disk of a system usually fall a victim to such kind of viruses. Overwrite virus removes the information possessed by the files either partially or completely making the important files profitless.

The only way to clean a file infected by an overwrite virus is to delete the file completely, thus losing the original content.

Examples of this virus include Way, Trj.Reboot, Trivial.88.D.

7. Macro Virus

Macro viruses usually create problems in the files that are created using certain applications or programs that contain macros.

Examples of macro viruses include Relax, Melissa.A, Bablas, O97M/Y2K.

8. Polymorphic Virus

It is quite complicated to detect Polymorphic viruses as this kind of viruses use encryption method when they infect a system. Thus it becomes impossible for anti-virus programs to find out these viruses.

Examples include Elkern, Marburg, Satan Bug, and Tuareg.

9. File Infectors

This type of virus infects programs or executable files i.e. the files with an .EXE or .COM extension. When one of these programs is run, the virus gets activated, thus producing the damaging effects it is programmed to carry out.

Example of file infector virus include Cascade virus.

10. Encrypted Viruses

Sharing the attributes of Polymorphic virus, this type of viruses also use "encrypted code technique" which unable antivirus softwares to detect them. The antivirus program usually can detect this type of viruses when they try spread by decrypted themselves.

11. Companion Viruses

Companion viruses can be considered file infector viruses like resident or direct action types. Once this virus gets into the system, it accompanies the other existing files in order to carry out its infection routines. Companion viruses can wait in memory until a program is executed or act immediately by making copies of themselves.

Examples include Stator, Asimov.1539, and Terrax.1069

12. Network Virus

Network viruses rapidly spread over a network. This type of virus usually finds its prey through a Local Area Network (LAN), and sometimes throughout the internet. In this way it attacks one computer and moves towards the next computer through the network.

Example of Network viruses include Nimda and SQLSlammer.

13. Trojan Horses

Though, it’s not considered as a virus, Trojan horse is an unwanted program that causes problems in the system. Unlike other viruses, Trojan horse does not possess the ability to replicate modules whereas it works quietly in the system, making your computer's security system weak gradually.

14. Logic Bombs

Generally Logic Bombs are not categorized as viruses. However; they also have unfavorable influences on our system as the program intends to harm the system by destroying the data on the computer under certain conditions. Antivirus software cannot detect logic bombs.

15. Worms

Technically, a worm is not considered as a virus, however, like virus, this program also have negative impacts on the computer system. It has the ability of self-replication as well. Like other viruses, this program is also detected by antivirus software.

Examples of worms include PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson.