Sobig-F has taken the record as the world's most rapidly spreading virus to date, according to managed services firm MessageLabs, which stopped more than one million copies of the email-borne nuisance since its first appearance earlier this week.

Sobig.F has surpassed the infamous LoveBug, Klez and Kournikova viruses.

Sobig-F, first detected on 18 August, is the sixth variant issued in the Sobig series and appears to be the most sophisticated to date, according to MessageLabs. All initial copies originated from the US, where the virus is currently most prevalent.

Since the first Sobig virus first appeared on 9 January, MessageLabs has intercepted almost three million copies of the virus' variants. MessageLabs detected all strains of this virus proactively, using its heuristics technology.

The current Sobig virus to email ratio is approximately 1 in 17 and the virus is spreading at such a rate it is expected to continue to stay at high-level status for the next few weeks. However, like past Sobig viruses, the Sobig-F virus has an expiry date and is set to deactivate on 10 September.

Said Mark Sunner, MessageLabs' CTO: "The Sobig virus writer's use of an in-built expiry date indicates that he is committed to inventing new and improved versions. Each variant released so far has exceeded the previous one in growth and impact during the critical initial window of vulnerability."

Sobig is a mass-emailing virus that can spoof the sender's address, fooling the user into believing the email is from a legitimate source and then opening the email. The email often contains the following header: "Subject: Re:details" and the text "Please see the attached file for details." The attachment names may include: your_document.pif, details.pif, your_details.pif, thank_you.pif, movie0045.pifm, document_Fall.pif, application.pif, docment_9446.pif.

Once the virus has infected your machine it attempts to connect to a website to download a backdoor Trojan, leaving your computer vulnerable to security breaches by crackers or other viruses. ®