Computer Virus Guide

What is a Virus?

A computer virus is a programme designed to alter the way a computer operates, without the knowledge or consent of the user. There are two key aspects of a virus:

  1. They are self executing. Typically, a virus will attach itself to another programme on your computer, so that it is activated when that programme is used
  2. They are self-replicating. Computer Viruses are designed to spread from machine to machine and across networks. To achieve this, a virus will usually copy itself to other programmes on a computer, before executing any intended tasks

There are a huge number of viruses in existence, carrying varying degrees of risk. Some are extremely malicious, with the ability to delete or damage files and programmess. Others are less destructive, but prove debilitating by jamming resources, causing systems to crash with consequent loss of data.

Some of the most well known viruses are:

  • MyDoom
  • Bugbear
  • Klez
  • Lovebug
  • Melissa
  • Bubbleboy
  • Code Red
  • Nimda

Some viruses will hit as soon as they reach a machine, and signs of infection are immediately obvious. Others can remain hidden until triggered by a future event. It is therefore vital that full and thorough recovery procedures are followed when an infection occurs.

Different types of virus

Although there is a large number of viruses, they fall into three main types:

  • Macro viruses use features within standard applications, such as Microsoft Word and Excel to perform unexpected tasks. For example, inserting unwanted phrases or figures
  • File viruses normally affect programme files and are usually transferred by disc, file transfer or e-mail attachments
  • Boot sector viruses infect parts of your computer that are used when it starts up, typically the floppy disc and the hard disc. In doing so, the very act of starting a PC will activate the virus

You may also have seen the terms worm and trojan horses used in the context of viruses. These are variations on viruseswith their own characteristics.

What is a worm?

A worm is a programme that is designed to replicate and spread throughout a computer system. It will usually hide within files (for example, Word documents), and distribute those files through any available network connections.

Worms are often used to drain computer resources such as memory and network access, simply by replicating on a large scale. In addition, worms sometimes delete data and spread rapidly via e-mail.

What is a trojan horse?

A trojan horse is a malicious programme, usually disguised as something useful or desirable. When activated, they can cause loss, damage or even theft of data.

The critical difference between a trojan horse and a virus is that a trojan horse cannot replicate itself. The only way that a trojan horse can spread is if you help it! For example, saving the programme from an e-mail attachment, or downloading it from the Internet.

Just because trojan horse programmes are not self-replicating, it does mean that they are any less destructive than a virus. Some common features of trojan horse programmes include:

  • Rounding (carving off small parts of payments from a large number of accounts or transactions)
  • Causing payment triggers (causing illicit payments to be activated)
  • Making configuration changes
  • Distributing security information
  • Providing unauthorised access paths (known as backdoors and trapdoors)

What are the characteristics of a virus?

Some of the most common characteristics of a virus are:

  • A sudden decline in PC or network performance
  • Playing a tune or displaying a message
  • Loss of files or data
  • Loss of partitions (organisation of disc space)
  • Unauthorised release of files, usually via e-mail

See what are the signs of virus infection? for further advice on what to look for if you are concerned about a possible infection. Remember that just because you have some signs of a virus, it may not in fact be the case. It might be a virus hoax.

How are viruses transmitted?

Viruses are usually disguised as something else. For example:

  • E-mails or e-mail attachments
  • Internet downloads
  • Internet pages
  • Software releases on disc or CD


You are at risk from a computer virus infection if your computer system is used to communicate with others, be it via a network, the Internet or e-mail. Given that these are now the main reasons that we are using computers, the risks are very real.

There is a lot that you can do to protect your organisation against the threat of virus attacks. For further advice see our Virus Prevention and Good Housekeeping sections.

If you are concerned about your own risk from viruses, check the Virus Risk page.

If you have suffered a virus attack, check the Virus Recovery page.

For general advice on information security, see our Information Security section under Business Topics.

This information based on Crown Copyright 2003
1 2 3 4 5 6 7 8 9 10

Leave a Reply

Your email address will not be published. Required fields are marked *


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>