If this Lex note gets through to you, count yourself lucky. The computer virus 'Zotob' attacked several global media companies on Tuesday, paralysing their systems. It seems a long way from 1987, when a student in New Zealand caused selected computer screens to flash “Your PC is now stoned”.

Symantec, a provider of anti-virus software, estimates that 92 per cent of US companies were attacked in 2003. Basic viruses infect legitimate programs with code. More complex 'worms' (such as Zotob) hijack control of software, such as email, to replicate themselves. 'Trojans' rely on human curiosity to spread, often by using saucy email headers.

Viruses certainly infect IT professionals with an uncharacteristic sense of purpose. But are they really a grave threat?

Academics affiliated with the University of California and the US Department of Defense estimate a 'worst case' US worm, launched with the support of a hostile nation state, would cost $50bn in lost output and clean-up costs. Its probability is impossible to predict. But the damage, at 0.4 per cent of gross domestic product, is not catastrophic.

Whatever the cost-benefit analysis, humans are price insensitive about infection. As attacks rise, the $10bn anti-virus software market is booming. Leaders Symantec and McAfee had annual pro-forma sales growth of 22 per cent last quarter, while Microsoft has made recent acquisitions in the area.

Indeed, if viruses did not exist, the IT industry might be tempted to invent them.