A virus disguising itself as an image of sexy Russian tennis star Anna Kournikova is clogging the email systems of companies across the globe. The email worm appears to be mutating as it spreads around the world, with several different subject lines reported for the emails through which it propagates. Experts believe that it will not affect Macs, but managers of cross-platform networks should beware.

The virus, known as Anna Kournikova or VBS_Kalamar.A, is a mass-mailer that uses Microsoft Outlook or Outlook Express to email itself as an attachment. Microsoft released a beta version of the Outlook email client for Macintosh at January’s Macworld Expo in San Francisco.

Windows only? Security vendor Trend Micro says it is spreading more rapidly than last year's infamous I Love You virus that affected an estimated 15 million PCs worldwide. The worm, likely to have originated in the Netherlands, only infects PCs running Microsoft Windows and can only propagate on machines running Outlook that have not installed the patch issued by Microsoft after the similarly-designed Love Bug virus swept around the world last year. Like the Love Bug, it mails itself to everyone in an Outlook user's address book.

However, Trend Micro marketing manager Andy Liao says that business need not be alarmed over the virus, as it can be easily blocked at the email firewall. "This is the most basic kind of virus. The only danger here is that it tricks people into running the attachment, exactly like I Love You, and it will spread via Outlook only."

Depending on the industry, blocking all email attachments or scanning and cleaning email systems promptly would ensure productivity levels wouldn't nose-dive, Liao advised.

Anna spotting The virus, as all but Computer Associates (CA) have termed the threat, features one of three variants of the subject line "Here you go :-)" as well as three variants of the name for the attachment, based around "Anna.Kournikova.jpg.vbs." The image is intended to appear to be a .JPG image of Anna Kournikova. The email resends itself, but does not appear to do any damage like deleting files or corrupting data.

"Damage is a variable term," CA business manager Ian Hameroff said. "This does cause damage in ways such as inappropriate bandwidth use or by filling up an email server."

The virus appears to be doing both ably.

Trend Micro says the virus should come as no surprise to businesses given that it hit the day before a universal event like Valentines Day. "Virus-writers try to target certain days and it's more likely that viruses will hit on significant days. I Love You was about two days before Mother's Day."

Vincent Gullotto, senior director of McAfee Avert, said: "There's no guarantees anymore what you're going to read in email or see in an attachment is what [an email sender] says they really are."