They attack without mercy. They cause damage worth millions. They mutate into new forms to outwit their pursuers. And, with the rise of the Internet, no one is immune. Stephen Pritchard investigates
Each year virus attacks cost millions of dollars. Information from the US research organisation DataQuest shows that 62 per cent of virus attacks result in lost productivity. Almost two-thirds of companies have been attacked by viruses, tiny programs that spread from computer to computer and cause malfunctions, from rude messages to disk errors. Recovery time for an average local area network is up to 30 days, and the costs of a virus attack can be up to $15,000 (pounds 10,000) per network.

According to Steve White, anti-virus expert at IBM, there are about 8,000 known viruses. Six more are written daily. The problem is not new, but the growth in computing and, especially, the Internet is giving the problem a new edge.

Phil Swallow, a partner at the IT specialists Andersen Consulting, says viruses are "becoming more of a way of life". The virus is no longer something most people have only heard about. "Most people have experienced them at least once in the last 12 months," Mr Swallow says.

One reason is that the Internet makes it so much easier to swap information between computers. The Net also means that the individual computer user at home, who might never think of sharing floppy disks, is as vulnerable as a busy network.

In the past, viruses spread slowly between machines. To "catch" a PC virus, users needed to copy software, say a game, from an infected computer on to a floppy disk, then copy it on to their machines and run it. Companies installed anti-virus software that checked floppy disks, and banned staff from using unauthorised software. Although some organisations suffered attacks, by and large these measures kept the problem under control.

Unfortunately, viruses - or at least the underground community of programmers who write them - found new ways to overcome these defences. The Word Concept virus, which appeared last year, posed a far more serious challenge. It attacked files, not programs. Swapping programs is not only bad practice but often illegal, too. Exchanging files - by disk, over a network or the Internet - is an everyday part of office life.

Concept uses Microsoft Word's own macro language - which allows users to automate functions - to turn files into templates that cannot be edited. It has been followed by dozens of macro viruses attacking Word and, most recently, the spreadsheet Excel.

According to Aled Miles, marketing manager at Symantec, which produces the SAM and Norton Anti-Virus packages, 49 per cent of all virus attacks are now of the Concept type. Concept also spreads more rapidly. Before the macro virus, according to Mr Miles, it took three or four years for a new virus to make the top-10 list of reported attacks. Concept achieved that position in just six months.

If this were not enough, virus writers are developing yet another new strain: the so-called polymorphic virus. Like a human virus, these mutate after infection in an attempt to outwit anti-virus packages. Some viruses are even said to attack the anti-virus software itself. "It mimics the natural world," says Mr Swallow. "Just when you think everything has been licked, a new mutation is found."

Anti-virus houses are responding in kind, with software that emulates the human immune system. IBM already uses this approach in its labs to find ways to detect and repair new viruses, and other manufacturers are working on similar projects.

Currently, relatively few polymorphic viruses are "in the wild", according to the experts. The anti-virus industry hopes it stays that way long enough to build the next generation of detection software. If they lose the race, the effects could be nothing short of catastrophic.

At IBM, Steve White cites the case of the Internet Worm virus, which infected thousands of PCs in a few hours, as a portent of what might come.

"It raises a dark picture of hundreds of new viruses that spread over the Net to your PC. You will not be able to get anything done," he warns. IBM wants to create an "immune system for cyberspace". If they fail, Steve White's view of the future is stark. "Cyberspace will stop working," he saysn