The E and G variants of Klez were by far the most common infections in July, according to lists released Thursday by antivirus software and services companies Central Command, Sophos and Kaspersky Labs.
The remarkably durable versions of the Klez wormearly this year and quickly munched their way into the record books, overtaking as the most computer pest in recent history.
Security experts have attributed Klez's durability to its capacity for disguise, fooling recipients into opening infected messages. Klez-created e-mail messages can contain any of several dozen subject lines, such as the popular "An excite game!" and randomlyan e-mail address to mask the true sender.
Virus writers have kept busy during Klez's rampage, however, as indicated by the presence of several new pests in July's Top 10 lists. Thevirus has spread through infected e-mail messages purporting to contain a vital Windows password.
"Frethem uses a simple psychological trick to reel users in," said Stuart Palmer, managing director at Sophos.
Also making the charts was Yaha, which began as typical mass-mailing worm but mutated with its "E" variant into a political tool that used infected PCs to launch rudimentary denial-of-service attacks against the main Web site of the Pakistani government.
Steven Sundermeier, product manager for Central Command, noted that the overall number of virus reported in July wasfrom the previous month.
"Whether this is due to an increasing awareness of malicious code or simply because more users are on vacation and away from their computers, it's a trend we hope will continue," he said in a statement.