Definition of virus in The Network Encyclopedia.

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What is virus (in computer science)?

A Virus is any piece of code that is deliberately written to cause damage or annoyance to computer users on a network. Tens of thousands of different “strains” of viruses have been detected over the years. The effects of these viruses range from harmless messages announcing the presence of an “infection” to malicious deletion of crucial system and data files. The first recorded PC virus was the Pakistani Brain virus detected in 1987.

Common categories of viruses include the following:

  • Boot-sector viruses:
    Viruses that infect the boot sector of a floppy or hard disk and execute when a system is booted, causing various kinds of damage, including totally disabling systems. Notorious boot viruses have included the Michelangelo virus and the Stoned virus.


  • File viruses:
    Viruses that reproduce by attaching themselves to executable (.exe) files. When the executable file is run, the virus code is executed, causing the virus to reproduce itself and (typically) damage user files in the process.


  • Polymorphic viruses:
    Viruses that evolve as they reproduce, causing their signature to change and making them difficult to detect unless the specific evolution algorithm is known.


  • Macro viruses:
    Increasingly popular viruses written in the form of macros for word processing and spreadsheet applications. When the macro is executed, the virus infects the system and typically results in loss of files.



Related types of malicious programs that are not strictly viruses because they do not reproduce can have similar effects. These include the following:

  • Trojan horses:
    Programs that masquerade as other programs and are typically used to steal credentials or other information from a user


  • Worms:
    Programs that invade memory to destroy files but are not disk resident


  • Logic bombs:
    Programs that are triggered when a certain event or sequence of events occurs


Many malicious programs combine the properties of viruses with one or more of these programs and are not easy to categorize.


To keep viruses from proliferating on your network, you can take the following measures:

  • Install top-quality virus-scanning software on each computer in your network, and keep their virus information files up to date.
  • Be sure that your anti-virus software scans for viruses in Hypertext Markup Language (HTML) files and attachments to Simple Mail Transfer Protocol (SMTP) e-mail messages if you have Internet connectivity.
  • Regularly perform backups of all important servers and include periodic archives in your backup schedule, since many viruses do not activate for weeks or months after infection.
  • Scan new computers for infection before bringing them onto the network.
  • Issue a company policy prohibiting users from installing any personal programs on their desktop computers. You might even disable their floppy drives, because infection via floppy is a common route to virus infection.

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