What is a Virus, a Trojan, Spyware, etc.?

Infected Computer

The Threats

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Computer Viruses Computer Viruses

There are a number of technically distinct types of malicious software that we tend to group together under the term "virus." The "true" computer virus infects files on the host computer and, as in nature, self-replicates, (creates copies of itself.) It may be destructive or it may just spread. Even "innocuous" viruses slow down computers and networks and cause degraded performance and crashes.

A computer virus is defined as a computer program file capable of attaching to disks or other files and replicating itself repeatedly, typically without user knowledge or permission. Some viruses attach to files so when the infected file executes, the virus also executes. Other viruses sit in a computer's memory and infect files as the computer opens, modifies or creates the files.

Some viruses display symptoms, and some viruses damage files and computer systems, but neither symptoms nor damage is essential in the definition of a virus; a non-damaging virus is still a virus.

There are computer viruses written for several operating systems including DOS, Windows, Amiga, Macintosh, Atari, and UNIX, and others. Most virus checkers detect more than 65,000 viruses, Trojans, and other malicious software. (Note: The preferred plural is the English form: viruses)

Definition taken from McAfee's Virus Glossary

For information about current viral threats, visit our Virus Warnings page.

Trojan Horse Programs Trojan Program

A Trojan is a software program that does not replicate itself but must be "invited" (usually unwittingly,) by the user onto their computer.

It may destroy data or steal passwords. It may use the host PC to perform a Denial of Service (DoS) attack on another computer, or it may simply take up memory and resources.

Computer Worm Worm

A Worm, like a virus, replicates itself between computers. Unlike a virus, it can do this without infecting "host" software. As with a virus, the effects of a worm can range from the catastrophically destructive to the rather annoying.

However, we will use "virus" as generic term for any such malicious code from now on.

Computer Hoax Computer Hoaxes

Hoaxes are not viruses, but are usually deliberate or unintentional e-messages warning people about a virus or other malicious software program. Some hoaxes cause as much trouble as viruses by causing massive amounts of unnecessary e-mail.

Most hoaxes contain one or more of the following characteristics:

  • Warnings about alleged new viruses and its damaging consequences,
  • Demands the reader forward the warning to as many people as possible,
  • Pseudo-technical "information" describing the virus,
  • Bogus comments from officials: FBI, software companies, news agencies, etc.

If you receive an e-mail message about a virus, check with a reputable source to ensure the warning is real. Visit our Virus Hoax page to learn about hoaxes and the damage they cause. Sometimes hoaxes start out as viruses and some viruses start as hoaxes, so both viruses and virus hoaxes should be considered a threat.

Definition taken from McAfee's Virus Glossary

Other Threats

Other unpleasant types of program that may find their way onto your computer are as follows:

Scumware Scumware

Scumware is software that changes what website's you view. Advertisers pay the manufacturers of this software to "hijack" Internet traffic and send it to their website. The TopText browser add-on from Ezula.com is perhaps the most prevalent such application. It is distributed as part of the seemingly innocuous Kazaa file-sharing package. Microsoft's own "Smart Tags" were withdrawn after allegations that they also constituted Scumware.

Spyware Spyware

Spyware or adware are terms used to describe programs that display uninvited advertising on your computer or track your actions and Internet use. They are usually disguised as useful applications to help persuade you to install them. The Gator Corporation is probably the market leader in this method of advertising. If you install one of their applications you'll soon notice advertising popping up all over your PC. Spyware also raises huge privacy concerns, as there is no way of the user knowing for sure quite how much information is being collected or how it is being handled. Whilst web sites publish privacy policies, the progress of effective enforcement methods is years behind the industry, so trust inevitably enters the equation. Many pro-privacy campaigners believe that even cookies - tiny information files stored on your computer by a website which may be used to select what advertising to display to you on a return visit, or even to store personal data - represent a threat to online privacy.

Unfortunately the argument is not clear-cut as cookies also provide a number of real benefits - with cookies switched off, for example, you would have to log in to every familiar web site such as Hotmail.com or Amazon.corn on every visit.

Genuinely malicious, but thankfully less prevalent, forms of spyware can also be used to log "key strokes" and steal information such as passwords from unsuspecting users.

pop-up windows Pop-ups

Pop-ups are not in themselves harmful, but tend to annoy or even enrage web surfers! In extreme cases, a "chain" of pop-ups can open so quickly that they crash your computer. Often, attempting to close a popup will only spawn three more. Many spyware applications also attempt to install themselves through pop-ups.

dialer programs Diallers

Dialers used to be the darlings of the online Adult industry. A surfer was offered "free" adult material to download. They actually ended up downloading a piece of software that closed their normal Internet connection and redialled it using a premium rate 'phone number. These programs only affect computers with dial-up modems installed and connected. Unsuspecting users could run up phone bills of thousands of pounds as they paid two pounds a minute or more to access the Web, often for months after downloading the software. Most web surfers are wise to this ploy by now, though many of the less-scrupulous and seedy sites still try it on.

Dialler programs have recently began to cause its victims problems again even though few people are still using dial-up. Even though the program is unable to connect, it ties up computer resources by frequent attempts to connect to a modem and blocks the computer from connecting by broadband.

anti-spam Spam

Electronic spam (not the tinned variety) is the sending of unwanted and unsolicited emails. Named after the repetitive Monty Python's sketch, spam is the electronic world's biggest problem. While most countries are trying to enact laws to make the sending of such emails an offence, it remains big business primarily because some people will buy spam offers. The best way to deal with spam is to NOT reply but delete immediately, set your email software to not accept emails from people you don't know or set it to filter emails according to set words (sex, viagra, money etc.) Unfortunately a fool proof method of filtering out spam does not yet exist.

If you have a website you might also want to consider linking to this antispam page as it adds a page of false email addresses to the trail. Email collecting programs (spam bots) will be sent into an infinite loop by following the link at the bottom of the page and will get more and more fake email addresses stuck in their databases.


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