Klez Is Still No. 1 WormKlez Is Still No. 1 Worm

Klez just became the most persistent worm ever, antivirus company Sophos said in its monthly update of the top 10 viruses and hoaxes.

With 13 months on the charts, Klez remains "a worm with staying power," Chris Wraight, a technology consultant at Sophos, said in a statement released Monday.

Nearly 14% of the viruses detected by Sophos were the Klez-H variation. Next was the W32/Sobig-A, which appeared about half as often as Klez. Other notables in the list include Avril, Yaha, Bugbear, and last month's most-talked-about virus, Lovgate.

Kaspersky Labs, another antivirus vendor, also tagged Klez as February's most prominent worm. According to Kaspersky's top 20, Klez ranked No. 1 by accounting for nearly 30% of all known viruses.

Sophos urged users to update their antivirus software and virus-definition files.

"People infected by Klez couldn't have updated their virus protection in quite some time," Wraight said. "Considering that it's now possible to automatically update your antivirus software every hour, there really are no excuses."