A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.

Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.


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Though they're not a living thing like you and me, computers can get "sick" from viruses, too. A computer virus is a software program designed to replicate itself and spread to other machines. In most cases, the program is "malicious," meaning its purpose is to cause the computer malfunction in some way In general usage, the term "computer virus" includes all forms of "malware," or malicious software.


Famous computer viruses

Virus Short Definition

1. ILOVEYOU - (2000) One of the most widespread and rapidly spreading viruses ever, the ILOVEYOU virus spread via e-mail, posing as an executable attachment sent by a friend from the target's contact list.

2. Code Red - (2001) IIS on Windows servers were the target of this virus. It also launched denial of service (DoS) attacks.

3. Nimda - (2001) Nimda used seemingly every possible method to spread, and was very effective at doing so. Nimda is notable for being one of the fastest spreading and most widespread viruses ever.

4. Melissa - (1999) The Melissa virus is notable because it is a Word macro virus. It cleverly spread via e-mails sent to contacts from the infected users' address books.

5. Sasser - (2004) Sasser exploited a buffer overflow and spread by connecting to port 445 on networked Windows systems. The chaos caused was possibly the worst ever, as systems restarted or crashed.

6. The Morris Internet Worm - (1988) The grandfather of computer worms, the Morris worm infected Unix systems and was notable for its "accidental" virulence.

definition computer virus

7. Blaster - (2003) Blaster exploited a Windows operating system vulnerability and let users know of its presence with a system shutdown warning.

8. SQL Slammer - (2003) This tiny virus infected servers running Microsoft's SQL Server Desktop Engine, and was very fast to spread.

9. Elk Cloner - (1982) Despite Apple's marketing that their systems are less prone to viruses that was not always the case. Notable as possible the first personal computer virus, Elk Cloner infected the boot sector of Apple II floppies.

10. Creeper - (1971) This is noted as possibly the first ever computer virus.

It infected computers on ARPANET. Mostly harmless, the concept of Creeper has infected the minds of rogue programmers through today.

Common computer viruses

  • 1. Encrypted Viruses – The encrypted virus is probably the most difficult kind of bug to detect and the most difficult to stop. You may accidentally have downloaded one of these bugs and before you know it, your entire computer can be infected. Many top virus protection programs miss encrypted viruses because these bugs use a different form of encryption every time. When the bug wants to run wild, it decrypts itself. In most cases, your virus protection can then identify it and stop it.
  • 2. Secret Viruses – These types of viruses will make changes to files on your computer, or completely replace files, but then try to trick your computer and your anti virus program into thinking that the originals are being used. Most advanced virus protection programs can stop these common computer viruses dead in their tracks.
  • 3. Time Delay Viruses – These types of viruses take a much slower, more disciplined path towards ruining your computer. Instead of instantly trying to take over your computer the moment you download them, they will wait and slowly infect files bit by bit. You may not have been online for days but then suddenly find yourself with an infection. These common computer viruses are the reason why you should run your virus protection every few days, just in case.
  • 4. The Anti-Virus Virus – Believe it or not, there are viruses out there that do nothing more than attack your pre-installed anti virus program in hopes of disabling it so other viruses can then be downloaded. This is why many people have a virus protection program as well as a separate anti-spyware or anti-malware program on their computer.
  • 5. The Multi-Headed Virus – This is one of the most nefarious bugs on the whole Internet. Not only are there parts of this virus that will attach themselves to . exe files on your computer, but it will also affect your computer’s start up so that you begin running the virus every time you turn your computer on automatically.
  • 6. The Misdirection Virus – This type of virus is downright scary. It has a built in subprogram that is made to give false readings to your virus protection software. You think you have a bug in one directory, when, in fact, the virus is busy harming your computer in a whole other area.
  • 7. A Cloning Virus – The cloning virus is an old fashioned type of bug. When you download it, it will quickly create duplicates for . exe files you have on your computer, hoping that you’ll click on it when you really mean to click on a healthy program you already have.
  • 8. The Author Virus – When you download a virus, it usually attaches itself to a program and then runs when you run that program. The Author Virus, on the other hand, finds an . xe file and actually deletes and rewrites code so that the program is changed. Few common computer viruses run this way since the level of virus needs to be so sophisticated.
  • 9. The Bad Penny Virus – The very first computer virus to ever hit the Internet was a Bad Penny virus. This is a bug that automatically passes itself on to everyone on a network or on the Internet unless something stops it. This was the whole reason why firewalls were invented.
  • 10. When most of us think of viruses, we think of PC’s running Windows software. However, there are a handful of bugs out there for the Mac.
  • 11. Rewriting Virus – This bug made a habit out of rewriting some of your most needed files, as well as filling up your hard drive with all sorts of invisible files you couldn’t normally see.
  • 12. The Melissa Virus – This was a bug that hit everyone, both PC users and Mac users. It would automatically email itself to other people without permission. It can be extra harmful if you use a private mail server at your place of employment. The Melissa virus has gone down in history as one of the most common computer viruses of all time.