Zombies join the attack
For years they've worked in separate corners of the electronic world. Virus writers exploited computers for fun or curiosity or out of sheer malice. Spammers tried to bypass email filters to place unsolicited commercial advertisements in email inboxes.
Now they're working together.
Infecting personal computers has evolved into a lucrative industry. The emerging alliance between spammers, virus writers and even organised criminals threatens to make personal computer users unwitting parties to online crime.
Formerly viruses carried a single hit or "payload" programmed to do anything from displaying an ironic or playful message on a user's screen to the more malicious actions of viruses such as the Gigger worm, which wreaked havoc with a hard drive. Once it had achieved its purpose, the virus would replicate and spread, having done its damage.
Now, however, a new breed of virus is taking a more sophisticated and destructive approach. Viruses now seek to embed themselves on personal computers, creating security holes on individual PCs and allowing them to be controlled by hackers over the internet.
"What we're seeing now is a change in what viruses are aiming to do," says John Donovan, the managing director of Symantec Australia. "A number of years ago programmers of malicious code would attempt to attack a system; now they try to compromise the security of a computer for personal gain."
Virus writers target unprotected home computers, looking to bypass their security systems and take control of computer activity.
"The issue is average home users don't have the level of protection they need to secure their computers," says Allan Bell of McAfee Asia-Pacific, which makes anti-virus software. "They don't see their machine as being a target for hackers. But just because they don't have valuable data on their machine doesn't mean that a hacker won't open up their computer."
The infected PCs or "zombies" are hijacked by internet criminals. "These zombie computers are used by hackers to then do further damage," says Bell.
Virus writers try to connect the PCs they've infected into networks or "botnets". Botnets usually consist of infected Windows PCs networked via the internet, ranging in size from 10,000 up to 100,000 PCs.
Once the botnet is established, virus makers can then lease access to email spammers. Sales are conducted via instant messaging programs and can fetch between $65 and $130 for an hour of access.
Spammers will then use these infected computers to send millions of unsolicited messages in just a few hours, preferring to use networks because it makes it harder for recipients to block the emails and it protects the spammer's identity.
The unprotected users whose machines are being harnessed often don't notice the activity, with most hackers concealing their intrusions. "This represents the biggest risk - people being infected but not knowing that they're being infected," Donovan says.
The chances of the average PC user becoming infected are increasing. Symantec estimates that Australia is fourth in the world in the level of PC virus infection, behind the US, Canada and China. The proliferation of viruses and the sophistication and power of virus code is attributed to an intricate global network of virus writers.
"We can see through things like the development of virus programs, from early scares like the  Michelangelo right through to the recent Sasser virus, that the newer versions have multiple additions to the code, making them more sophisticated and harder to detect, which points to a fairly well-developed network of people who are developing [the viruses]," Donovan says.
Authorities are struggling to keep track of the fast-growing criminal industry. Virus writers have a diverse physical presence, with hot spots ranging from Russia to Indonesia and even the US. The only step that can be taken against the mounting threat is an awareness of the risk. "The challenge is not a lack of anti-virus technology; it is understanding how to use that technology," Donovan says.