Some viruses posed as messages from Microsoft

The number of viruses so far this year is up by nearly 20%, compared to the same time last year, according to figures from anti-virus firm Sophos.

The most prevalent bug was the Bugbear.B worm, even though it only appeared just a few weeks ago.

The virus spread so fast because it used different methods to trick people into opening infected e-mails.

Blended threats

Over the past six months, virus writers have become increasingly inventive in disguising computer bugs.

Worryingly, they have used different channels to spread a virus, such as e-mail, file-sharing programs and internet chat software.

Anti-virus experts Sophos said it had detected 3,855 new viruses in the first half of the year. This amounts to a jump of 17.5% over 2002.

TOP TEN VIRUSES Bugbear.B - 11.6% Sobig.C - 9.7% Klez.H - 8.4% Sobig.B - 5.3% Sobig.A - 3.3% Avril.B - 3.2% Bugbear.A - 2.5% Avril.A - 2.3% Fizzer.A - 2.3% Yaha.E - 1.8% Source: Sophos

The most widespread was Bugbear.B, which appeared at the beginning of June. In just 24 hours, the worm spread to 159 countries.

It was what anti-virus experts call a "blended threat" - a bug which contain several techniques for spreading and infecting computers.

"Bugbear.B entered the frame late, but nevertheless it has generated more enquiries that any other virus in the last six months," said Graham Cluley, Sophos Senior Technology Consultant.

"By morphing its contents every time it forwards itself, and by spoofing the e-mail address of the person who sent the virus, Bugbear.B has been the most prevalent and irritating virus so far this year."

Other popular viruses hide in messages pretending to be from Microsoft.

The second most common virus was Sobig.C, which appeared in early June posing as an e-mail from Bill Gates. The previous month another virus was disguised as a message from Microsoft support.

Avril bug

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Virus writers are always on the lookout for ways to trip up unsuspecting computer users and disguising a worm as a message from the world's best known software firm is the latest in a line of cunning tricks.

Another technique is to use a celebrity name. So far the virus celebrity of the year is the Canadian teen sensation Avril Lavigne, with two Avril bugs making up 5.5% of all virus reported to Sophos.

But attempts by virus writers to capitalise on interest in news events appear to have backfired. Viruses which used the Sars outbreak and the war in Iraq both failed to cause widespread infections.

As a rule, computer users should keep their anti-virus software up to date and be wary of e-mails from unknown sources.