Just who is responsible for releasing these plagues, some of which can destroy a computer's hard drive and render the machine useless, is a matter of some debate.

Global reach: A virus can spread worldwide

Conspiracy theorists dismiss the notion that misguided teenagers are responsible, preferring to blame various governments and even canny makers of anti-virus software.

One thing for certain though is that viruses come at a heavy cost. Research by Computer Economics found that virus attacks cost business more than $12.1bn in 1999.

Much of that would have been down to the best-known virus of all - Melissa. Released in March last year, Melissa is estimated to have caused $80m damage.

The virus is self-propagating - it hijacks a computer's e-mail program and automatically copies itself to the addresses stored within.

The burden of having to process thousands and thousands of e-mails brought to a grinding halt the internal communications systems in several major companies.

The Chernobyl virus caused computer meltdown

The Chernobyl virus hit Asia and the Middle East last year when it was unleashed on the 13th anniversary of the Soviet nuclear accident. Precious files were lost as the virus wiped hard disks and some computers were even robbed of the ability to be turned on.

When it was unleashed late last year Bubbleboy heralded a worrying new trend - that of the virus worm.

Whereas most e-mail viruses are only triggered when the user opens an attachment, worms will activate in other ways.

With Bubbleboy - named after an episode of the TV comedy Seinfield - clicking the e-mail's subject line in Microsoft Outlook is enough to set it off. The virus also takes every address in a computer's e-mail program and passes the virus along.

Work and play: All computers running Microsoft Outlook are vulnerable

Laroux was named as the most prevalent virus in 1999 by anti-virus software firm Sophos. The virus is a macro - a tiny program within a program - which affects certain versions of Microsoft Excel.

Once a user's version of Excel is infected, any worksheets created using that release will contain the virus.

Its effect is to delete legitimate macros and so is considered to be not too serious.

Billed sometimes as the original computer virus, Cascade was detected in 1988. The original version caused characters to "fall" from the screen.

Although more than 10 years old, users should be aware that computer viruses do not necessarily die out. The key to a long-lived virus is invisibility. While highly destructive ones draw attention, more innocuous ones can lurk for years without being detected.

David L Smith was accused of creating Melissa

An example is Form, which originated in Switzerland and is triggered on the 18th of every month. One of the most common viruses of recent years, it does little damage other than to use up system resources.

Even less potent is Wobbler - one of dozens of hoax viruses that circulate by e-mail. Like E-flu, Irina and Hitler, Wobbler has only ever "spread" in e-mails warning of its existence.