Virus writers are setting up peer-to-peer networks to help their malicious creations spread.
Many computers are under the remote control of virus writers
The networks are being used to control thousands of innocent PCs that some virus programs have infected.
The tactic is being used because peer-to-peer networks are hard to disrupt, making viruses using this technique hard to stop spreading.
Security experts say peer-to-peer networks are likely to become more and more popular with virus writers.
One of the first viruses to set up a peer-to-peer network to help it spread was the Slapper worm that was aimed at the Linux operating system.
A Windows virus called Sinit appeared in late 2003 that turned every machine infected by the malicious program into a member of a peer-to-peer network.
It was expected that Sinit's creator would issue commands to infected computers via this network.
"It's always been an arms race in the battle between virus writers and anti-virus companies," said Pete Simpson of mail filtering firm Clearswift.
"Sinit represents a new and daunting challenge to anti-virus companies."
There are many different ways to organise machines into peer-to-peer networks but most avoid having one central computer, or server, controlling all machines involved.
Instead, each machine shares the burden of organising the system and each member lets others know the information that it holds.
The most well-known peer-to-peer networks, such as Kazaa, are used by many people to find and share music, videos and other types of files.
In the past some creators of Trojan programs, that open up a backdoor into an infected PC, have used net chat channels as a way to issue commands.
Often thousands of computers were enrolled in these remote controlled networks that have been dubbed "'bot nets".
Many people use peer-to-peer networks to swap music and movies
Finding and shutting down the chat channels would effectively cut a virus writer off from his network of slave machines.
But shutting down a distributed network would be much more difficult because no one machine is in charge.
It also is much more difficult to trace where commands were being inserted and find the network's controller.
Kevin Hogan, head of Symantec Security Response, said the good news about peer-to-peer virus networks was that they were rare.
"It's a little bit trickier to do than just having the Trojan point to a single server," he said.
He said many peer-to-peer networks were often not very efficient at passing commands between member machines. Also many swap data via rarely used ports that most firewalls routinely block.
"I think it will become more prevalent," he said. "It's an evolution rather than a revolution in bot nets."