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Kaspersky Labs said that the normally danger-fraught August - which last year saw PC users running the perils of SoBig, Blaster and Mimail - turned out to be an unusually quiet month.

Sophos put the Zafi.b virus as the most prolific for the month with 47.4 per cent of reports, followed some way behind by a litter of NetSky variants.

'Although we have seen a small, ten percent, decline in reports of Zafi-B since last month, this email-aware worm doesn't look like it's going to fade into obscurity anytime soon,' said Carole Theriault,

Kaspersky's chart had a more balanced feel, with a trio of NetSky variants taking the top three slots with figures in the high teens, followed by the Zafi.b virus with 7.5 per cent.

Despite the soft month, there were plenty of new viruses launched: Sophos identified more than 1,200 - the highest for nearly three years. Kaspersky highlighted two of them as particular dangers, creeping into its top 20.

It warns of two Trojans being spammed out - TrojanDropper.VBS.Zerolin and Downloader.Agent.bq - which, once executed, attempt to download other components, including keylogging utilities, installing backdoors and other viruses and spyware.

With the strong relationship now between spammers and virus writers, infected mails are becoming ever more prevalent. Sophos now says an average of 9.3 per cent of emails sent during August were infected.