Three days after its appearance, the Bugbear virus is still gaining momentum and has passed the venerable Klez virus as the most active in the past 24 hours.

One company that tracks viruses, Message Labs Ltd., said it has seen more than 21,000 copies of Bugbear in the last day, nearly twice the number of Klez-infected messages it's seen during the same period. Klez, however, is still by far the most active virus of the month and year so far.

Bugbear first hit the Internet late Sunday and has been spreading steadily ever since. In fact, F-Secure Corp., a Finnish anti-virus company, on Wednesday upgraded its alert level on Bugbear to Level 1, which is reserved for epidemic-level viruses such as Loveletter and Nimda.

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