Antivirus companies have named Klez as the most prolific virus of 2002.
UK-based Sophos revealed that the worm accounted for almost a quarter of reports to its customer support department this year, and topped the company's monthly chart for seven months in succession.
The second most common virus was Bugbear, which made the number two slot even though it was only detected in October.
"Unlike previous chart toppers like the LoveBug, which disappeared almost as quickly as it arrived, Klez is the ultimate in slow burning worms," said Graham Cluley, senior technology consultant at Sophos.
"It has managed to consistently infect users throughout the year. Protection against Klez has been available for as long as the worm has been circulation.
"The only possible explanation for its continued 'success' is that some users are habitually neglecting to update their antivirus software."
Despite Klez being the most active worm, antivirus firm Messagelabs said that Bugbear can lay claim to being the most dramatic outbreak of the year.
The company said that it was stopping one in every 87 emails at its height in October. Klez could only reach one in every 169 even at its peak.
Alex Shipp, senior antivirus technologist at MessageLabs, said: "A ratio of just over one in every 200 emails proves that 2002 has seen a major rise in the number of viruses in circulation, even if we haven't seen the dramatic outbreaks of previous years.
"The more prevalent viruses this year have been the ones most people have found hardest to spot, like Klez and Bugbear.
"This is because these are able to 'spoof' email addresses, so that the identity of the real sender is difficult to trace.
"It also means that, by mass mailing contacts from a recipient's address book, further victims are likely to open the rogue email because they think it is from someone they know and trust."
Both MessageLabs and Sophos indicated a worrying increase in the number of new Trojan horses being created.
"We have noticed that there is a shift from creating new viruses to the virus writers bringing out new Trojans," said Shipp.
"We have intercepted these being sent to big companies. Presumably, if they were hit, this would be a feather in the cap for the virus writers."
Clulely added: "We are generally seeing an increase in the use of Trojan horses to break in and steal passwords."
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