The MyDoom worm first hit the web way back in January 2004, but security experts claim that its effects are still being felt online.
MyDoom initially targeted software company SCO after infecting thousands of PCs worldwide and launching a distributed denial-of-service (DDoS) attack on the firm's website.
Microsoft and the RIAA were also targeted by MyDoom.
Having spread through file-sharing networks such as Kazaa, it was estimated that one in every 12 emails sent were carrying the worm in the days after it was first unleashed.
Incredibly, security firm MessageLabs, now owned by Symantec, says that it still encounters emails carrying the worm.
Paul Wood, senior analyst at MessageLabs said: "To this day, we are still stopping MyDoom infected emails, thankfully not on the same scale as 2004 but still in many different variants or disguises.
"MyDoom continues to hold the ominous position of being the fastest spreading email-worm ever known, and its emergence five years ago created widespread concern over the security of email transactions," he continued.
However, Wood said that outbreaks on the scale of MyDoom were much less likely these days.
"People are now much wiser to the risks of opening email attachments so it is less likely that an outbreak could now occur on the same level," he said.
Despite this, though, the Downadup worm is currently spreading to millions of PCs worldwide.
www.messagelabs.co.uk www.symantec.com