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But Kaspersky described the virus as 'a paradox - an average worm, with nothing interesting in the code or the social engineering methods used to trick users into opening infected attachments. And yet it has beat many more technologically advanced viruses. Certainly changing the language of the incoming email in accordance with the recipient's country is a novel idea. However this is Zafi.b's only interesting feature. Perhaps Zafi's dominance

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can be explained by the fact that users have relaxed now that summer is in full swing and are being less cautious about opening attachments.'

The rest of the charts saw little change, populated with the now familiar fayre of Netsky, Bagle and MyDoom variants.

What is more worrying perhaps is the number of new viruses that are being discovered. Sophos counted an extra 1,157 in July. Kaspersky noted more than 1,000 additional strains as well - a threefold increase over June.

Sophos says viruses now make up more than 9 per cent of the email it scans, while MessageLabs, which handles significantly more email through its systems puts this figure at 7.5 per cent.

However, this is dwarfed by MessageLabs spam figures. Of the billion plus emails it scanned in July, it found 94.5 per cent - that's 19 in 20 emails - were binned as spam.

This is perhaps indicative of the increasing efficacy of spam filtering software - that spammers have to increase the volume they send out to get the same returns.