COMPUTER 'worms are set to become a more deadly combination of virus writing and hacker exploits, according to security experts at Symantec.

Code Red and Nimda marked the demise of socially engineered 'worms,' by combining a blended threat of proven hacker exploits.

Both worms attacked the same buffer-overflow vulnerability in Microsoft's IIS software while Nimda additionally incorporated a mass-mailing component enabling the virus to propagate on a massive scale.

Neither of the 'worms relied on the traditional need for an infected computer user to double-click on a malicious attachment.

"Nimda and Code Red have eliminated the need for human intervention, by virus writers using what hackers have already provided," said Eric Chien, chief researcher at Symantec.

"One year ago email 'worms were the big threat, as they spread quickly and far -- but now a lot more virus writers will be looking at the hacker worm."

Chien predicts that by next year, the "blended" threat of computer 'worms' could be enough to cause a serious Internet slowdown.

Matt Loney, director of News, ZDNet UK, said: "Most people think they're safe from viruses so long as they don't actually try to open or run an email attachment, or download a program from the Internet that does not come from a trusted source. This was once the case, but no longer.

"As computer users get wiser to the threats, virus writers are turning to ever more devious means to spread their viruses. PCs could be infected with the Nimda virus simply by browsing a Web page that was hosted on an infected server. In fact, Nimda was the first massive virus to use a whole range of methods to spread."