Computer viruses are of the three common types – Worm, Trojan, and Bomb. Let us study these virus types.


Worm: Worm is a virus that spreads by using e-mail. They look in the e-mail address book of a user and send themselves to all the addresses stored therein to infect them as well. Worm is normally found n networks than on stand-alone systems as they get a chance to quickly spread to a  lot of computers, using the network as a medium. Examples are Scalper, So big and Swen.

Trojan: Trojan is a virus that hides some destructive program in a useful appearing program. For example, a program that shows crackers on your screen or offers a drawing tool may delete some of your files, working in the background by being active in the memory. Most of the current viruses contain both Trojan and worm so that a user looking program may delete important files and data as well as replicate, using network. Examples are Backdoor, W32. Azak and Nuker.

Bomb: It is a virus that becomes active when a specific event occurs and destroys or corrupts the document and program files on a computer. For example, Jerusalem-B waits for Friday the 13th and deletes the program files executed on that day.

Viruses are also classified on the basis of the infection caused by them. These categories are –

MBR Infector: Master boot record infector virus infects the master boot record of the hard disc. The MBR contains very important information regarding boot files and is read every time when a computer boots from a hard disc and thus the virus resident in the MBR is loaded by BIOS. An example of an MBR virus is – Cox.

DBR or Boot Sector Infector: DOS boot record infector virus spreads through infected floppy discs and hard discs. Whenever an infected floppy or hard disc is boot a system, the virus becomes active and starts spreading and infecting other computers. Example – Form

Program File Infector: Another type of virus that does not infect MBR or DBR but attaches to some executable programs such as.COM, EXE, .SYS, OVL, etc. These types of viruses turn the useful programs into Trojan programs and when these infected programs are executed, the viruses become active in memory and start spreading.

Examples of this type of virus are Friday the 13th, Tuesday the 13th, Sunday, Cascade, Nemesis, Dark Avenger, Loki, etc.

Multipartite Virus: A virus that contains program codes of MBR, DBR, and File Infector in its program is known as a multipartite virus. Examples are BootEXE and EXEBug.

Stealth Virus: This type of virus keeps on changing its program code so that it doesn’t get detected by a virus scanner. IT be fools the virus scanner by making it check the uninfected program and not the infected program, which is copied somewhere else beyond the antivirus software or virus scanner’s detection Examples are Frodo and Brain.

System Infectors: Viruses that infect the OS software partly or completely. Once the OS files are infected, the computer doesn’t boot. They destroy or delete themselves once the intended damage is done. Examples of System Infector viruses are Stoned, and AntiCMOS.

General Purpose Application Infectors: These types of viruses can infect any application program such as word processors, database management programs, accounting programs, presentation programs, etc. They infect the corresponding program files with extensions like .com, .exe, .dll, .scr, etc. They spread very fast by attaching themselves to the programs loaded in the memory.

Examples are Scores Virus and Israeli Virus. Macro Virus: Infect data files such as MS Word document, i.e., .doc files and template files.  They spared quickly when the infected files are shared through the Internet and networks such as LAN and WAN, etc. Examples are Melissa and Babylonia

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