I've always been complacent about the dangers posed by myxomatosis. I'm not a rabbit, after all, and I possess few rabbit-like characteristics other than a fondness for carrots, so I've never felt the urge to find a vet deranged enough to vaccinate me. Mac users tend to have a similar attitude towards computer viruses, figuring that it's simply not their problem – and this view has been tacitly supported by Apple in the past, with advertising campaigns that have emphasised how impervious Macs are to malware. While geeks will debate endlessly whether Macs are inherently more secure, the undeniable fact is that most hackers just haven't bothered attacking them because they represent fewer than 4 per cent of computers worldwide. After all, if you were hell-bent on sending a robot to wreak havoc on, say, Britain's food outlets, you'd program it to head for branches of Tesco rather than Ted's Food-Mart.

Companies like Symantec and McAfee have tried relentlessly to sell Mac antivirus software for years, and often talk up the threat posed by Mac viruses. But the number of Macs that have ever succumbed to serious infection is comparatively minuscule, and surveys have shown that fewer than 10 per cent of Mac users even bother installing such software. However, as Apple's market share increases, it's more likely that Mac malware will spread. Because whatever they may think, Mac users are just as likely to fall for the same social engineering tricks – clicking on dodgy weblinks, opening email attachments without thinking – as their Windows-using cousins.

The question is when the threat might be worth taking seriously. There is a Mac virus currently circulating – brought to our attention by Symantec – that lurks in dodgy copies of iWork and Photoshop CS4, works its way under the bonnet of your machine and has the potential to connect with other compromised Macs to form some kind of evil botnet. But if you're not a "high-risk" user who downloads material whose provenance is unclear, it's unlikely to affect you – so the decision of when, if ever, to buy antivirus software is analogous to the one about how long you're prepared to hang on to a rising balloon. Symantec and McAfee would say that putting off that purchase is a false economy – but the vast majority of Mac users will only get our their credit cards until they've fallen victim. If, indeed, they ever do.

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