The SoBig.F outbreak, first detected Monday, began 10 days after the Blaster worm (which itself infected an estimated 500,000 users) and has already beaten other infamous viruses such as LoveBug, Klez and Kournikova in terms of spread. Second wave attack also predicted.

The first SoBig F variant was released in January.

"This is the most severe e-mail virus we've ever seen," MessageLabs' Josh White said. "At its peak 1 out of 17 e-mails that we were processing was a copy of the SoBig.F virus. Certainly we haven't seen numbers like this before. It is spreading at a very fast rate and the volumes are high."

The e-mail-borne worm arrives with various subject headers, such as: Your details, Thank you!, Re: Thank you!, Re: Details, Re: Re: My details, Re: Approved, Re: Your application, Re: Wicked screensaver or Re: That movie.

The body of the message is short and usually contains either "See the attached file for details" or "Please see the attached file for details."

Fooled that the e-mail is legitimate, the user opens the e-mail and triggers the worm, which then goes hunting for addresses. The flood of messages it then sends are capable of succumbing other users' inboxes or computer systems by the sheer volume of e-mails.

The virus also implements a background program that turns an infected computer into a relay system for further messages from the virus' creator.

This part of the virus has led many computer security experts to believe the virus was written to try and beat spam filters.

Experts are predicting that though it will soon be brought under control, the infection is likely to spike early next week as many people in Europe and the U.S. return to work from (northern hemisphere) summer holidays to awaiting e-mail inboxes.

Unlike MS Blaster, this worm can only arrive via an executable email attachment. If you have up-to-date anti-virus software and the latest downloads, you should be safe.