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The oft-repeated mantra that Mac OS X is safer from malware attacks than Windows is actually true. To gain control of your system, viruses and Trojan horse programs typically need to hijack low-level OS functions. Before Vista, this was pretty easy to do on Windows. But Unix-like systems -- including Mac OS X and Linux -- make it hard for malware to muck about with their internals, because software does not run with administrative privilege by default. It's as if there's a firewall in place between your applications and the important parts of the system.
Popular wisdom also says that Macs are not good targets for viruses because Apple's market share is so low. This is also true. Like real-world viruses, computer viruses can't spread very well when they don't encounter other computers to infect. Thus, more viruses are written for Windows -- which has the most market share -- than for Mac OS X.
But that's not to say Mac users should be complacent.
Curious that the author didn’t reiterate/elaborate on the “before Vista” part of the story – I suppose saying potentially negative things about Apple and potentially positive things about Microsoft in the same article would have violated some natural law of the universe…