Like sleazy one-night stands, most e-mail viruses depart soon after they have had their way with their hosts.

But Klez seems to have decided to establish a long-term relationship with Internet users.

Klez, dubbed the world's most pervasive e-mail virus last May, is now also the most persistent Internet pest ever, according to representatives from antivirus firms Nod32, Sophos, Kaspersky, MessageLabs and Central Control.

"Klez is hanging in there like a bloated tick," said Rod Fewster, Australian representative of antiviral application Nod32. "We probably won't see the end of it in our lifetimes."

There are several variants of Klez floating around, but "Klez.H," the one that shows up most commonly in e-mail inboxes, has topped most antiviral company's threat lists since it was first spotted in mid-April 2002.

Antiviral companies often chide users for not updating their antivirus software, but some experts said Klez proves that repeating stern update warnings ad nauseum isn't going to solve the problem.

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