Computer Virus


Welcome to class! 

In today’s class, we will be talking about computer virus. Enjoy the class!

Computer Virus

computer virus |

Computer virus is a software program written with malicious intentions. They are intentionally written by some software developer to cause severe damage for the computer, programs, and data files in which it found itself. A computer virus has the ability to replicates itself by changing other programs to include a copy of itself.

Examples of computer viruses include form, cascade, virus 101, phoenix, stimulate, involuntary, proud, disk killer, multipartite, flip, Trojan horse, sleeper, logic bombs, Alabama virus, Christmas virus, worm, data crime, Jerusalem virus, April 1st virus etc.

Type of computer virus

There are basically three ways in which the virus will infect micro-computers:

  1. Boot sector
  2. Executable file viruses
  3. Document
  • Boot sector virus: They affect hard drives and flash disks by making itself available on the boot sector of the disk. Booting up from an infected flash disk can cause damage to the hard drive. Examples of boot sector viruses are polyboot.B, AntiEXE
  • Executable file viruses: They spread infection by attacking the code to executable files (EXE, COM) in this way, the virus program will be executed before the host program is executed.
  • Document virus: Virus can affect the document by moving from a disk partition table to a different sector and replace with its own code, therefore. Infecting document as they are accessed.

Other types of virus include the following:

  • Resident virus: This type of virus dwells in the RAM memory is permanent in nature. Its effect of existence is to overcome and interrupt all the operations executed by the system. For instance, corrupt files and programs that are opened, closed, copied etc.

Examples are CMJ, Meve, Randex, and Marklunky.

  1. Micro Virus: This type of virus infects files that are created by making use of a certain program or application that have macros. Examples are Relax, Melissa. A Bablas, 097M/Y2K
  2. Overwrite Virus: Overwrite virus deletes the information contained in the files, it duplicates itself by rendering information partially or totally useless. The only way to clean a file infected is to delete the file totally, thus losing the original content.
  3. Directory Virus: This virus changes the paths that indicate the location of a file. When you execute a program that has been infected with this virus, indirectly you are running the virus program, and meanwhile, the original file and program has been displaced by the virus. It is then impossible to locate the original location of file or program.
  4. Polymorphic viruses: They change themselves through different Encryption and this act make it impossible for antiviruses to find them using string or signature searches. They have the ability of duplicating themselves in large numbers. Examples include: Elkern, Marburg, Satan Bug, and Tuareg.
  5. Fat virus: This is part of a disk that is used to connect information and it is a vital part of the normal function of the computer. They prevent access to a certain section of the disk where important files are stored and the damage caused can result in information losses from individual files or entire directories.


  1. Define computer virus.
  2. State five types of computer virus.

Other malicious programs

There are other types of destructive programs not purely classified as virus but are equally threats to internet e.g. Worm

  • Worms:

Worm is considered to be a sub-class of a virus. It spreads from one computer to another without any human action and they have the capability of reproducing itself on the system at any point in time. For example, worm can spread itself to everyone listed in your email address book. Worm consumes or occupy a large part of computer memory thereby causing network servers and individual computers to stop responding. Worms travel across the network. Examples are, PSWBBugbear.B, Lovegate.F, Trile.C, Sobig.D, Mapson, Blaster.

  • Trojans or Trojan horses:

Trojans do not reproduce themselves by infecting other files nor can they replicate like a worm but they will actually do damage once installed in the system. Sometimes they appear to be useful software but when it is activated the result can be annoying (adding silly active desktop icons), or deletion of files and information on the system. Hacker utilizes viruses or Trojans to gain access to your computer, to steal data, invade privacy or use the computer for their evil purposes. e.g., Netbus, subseven, Beast, Zeus, Flashback Trojan, ProRat, ZeroAccess etc.

  • Logic bombs:

They are not really a virus parse because they lack the ability to replicate themselves. They can camouflage segments of other programs, even though they are not programs. They aim to destroy data on the computer once certain conditions have been met.

  • Adware and spyware:

They usually come together with other programs that seem to be harmless. These programs are not as dangerous as a virus but also collect data of your computer and send it “home” which is in most cases a company uses this information to spam you with advertising E-mails. Spyware can track internet searching habits and possibly redirect web site activity. e.g. The red sheriff and BESS.

Examples of computer virus
  1. Macro viruses Nuclear, DMV, Word concept
  2. Boot viruses Form, Disk Killer, Michelangelo, Stone virus
  3. Active X and JAVA viruses
  4. Stealth viruses Frodo, Joshi, whale
  5. Program viruses Cascade, Sunday
  6. Polymorphic viruses- cascade, EvilVirus 101, phoenix, stimulate, involuntary
  7. Multipartite viruses, Tequila, invader, Flip
General evaluation
  1. Define computer virus.
  2. State five types of computer virus.
  3. Mention six examples of computer virus.


Understanding Data Processing for senior secondary schools by Dinehin Victoria pages 287 – 289


  1. …………is a software program written with malicious intentions. (a) Computer virus (b) infection (c) e-scan    (d)  Antivirus
  1. ………… deletes the information contained in the files. (a) Fat virus (b) micro virus (c) Overwrite virus        (d) Polymorphic virus
  1. ……… utilizes viruses or Trojans to gain access to your computer, to steal data, invade privacy or use the computer for their evil purposes. (a) Hackers (b) files (c) human  (d) detector.
  2. Examples of computer virus include the following except …………….. (a) disk killer (b) e-scan (c) tequila (d) cascade
  3. A virus that can track internet searching habits and possibly redirect web site activity is called …………. (a)  logic bomb  (b) fat virus  (c) spyware  (d) worm


In our next class, we will be talking more about Computer Virus.  We hope you enjoyed the class.

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