Klez has been branded the most persistent worm ever to plague internet users, after spending a record 13 months in antivirus vendors' top 10 charts.
Chris Wraight, technology consultant at antivirus firm Sophos, said Klez victims had "no excuse" for not updating their antivirus software.
"In its various guises, Klez has now been hanging around the chart for 13 months, making it the most persistent worm ever," he said.
"People infected by Klez cannot have updated their virus protection in quite some time."
Klez was the most frequently occurring infection in February, accounting for almost 14 per cent of the month's total incidents, according to figures from Sophos.
The research also found that 541 previously unknown viruses, worms and Trojans emerged during last month.
Wraight expressed surprise that the much-hyped Lovgate worm was relatively insignificant, rating only ninth in the top 10.
"The new Lovgate worm may have been the most talked about virus in February, but it certainly wasn't the most prolific, reaching only number nine in the chart," he said.
Virus hoaxes continue to be a nuisance to users, with 'JDBGMR' the most common.
"This needn't be the case," advised Wraight. "People receiving this message should simply delete it and not be tempted to forward it to all their contacts 'just in case' it is a real virus warning."
For February 2003, the virus chart is as follows, with the most frequently occurring virus at number one:
1. W32/Klez-H (Klez variant) 13.7%
2. W32/Sobig-A (Sobig worm) 7.7%
3. W32/Avril-B (Avril variant) 6.0%
4. W32/Yaha-E (Yaha variant) 4.6%
5. W32/Bugbear-A (Bugbear worm) 4.3%
6. W32/Avril-A (Avril worm) 3.1%
7=. W32/Klez-E (Klez variant) 2.4%
7=. W32/Yaha-K (Yaha variant) 2.4%
9=. W32/Lovgate-B (Lovgate variant) 2.1%
9=. W95/Spaces (Spaces virus) 2.1%
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