Klez is the worst virus in the world. Ever...

The Klez worm has now overtaken SirCam as the most prolific virus of all time, according to anti-virus firm MessageLabs.

MessageLabs registered 776,725 instances of the Klez H-variant as of 11:00(BST) this morning - 14,000 more than the SirCam worm which has been plaguing networks since July last year.

The H-variant of Klez was first discovered just one month ago.

While neither SirCam nor Klez caused quite the same levels of damage as earlier email worms such as Melissa and LoveBug, both SirCam and Klez have proved difficult to eradicate.

Conversely, most successful viruses have a quick "spike" of popularity and then rapidly fade away. Klez managed to have a large spike of infections and keep going.

According to MessageLabs, the Klez worm now accounts for one out of every 300 emails sent worldwide, with 20,000 intercepted every day.

Like SirCam, one of the reasons Klez has been so successful is because there is no familiar title, message line or text for users to look out for.

It also fakes the sender's address, meaning it is very difficult to tell where the worm has come from, stopping people from informing others who unknowingly have been spreading the worm.