The reasons for this have been extensively debated by security experts, who offer several explanations:
?Small market share. There is some truth to the "security through obscurity" argument. Many virus writers are motivated by the power they can command -- and the money they can make -- by seizing control of large numbers of computers. That puts a financial premium on Windows viruses.
?Mac OS X, with its Unix-based file system and kernel, is harder to infect with a self-replicating program. (See Claudiu Dumitru's MacOS X Vulnerabilities for background.) Windows, as I understand it, allows users to write run executable code outside their own protected memory space; Mac OS X does not.
?Viruses are going out of style. The action these days, I'm told, is in Trojans and spyware.
This is not to say that OS X is invulnerable. The frequency of Apple's security updates and the emphasis the company is putting on the new security features in Snow Leopard are proof that it is not. Maybe Apple is just lucky. Or maybe it's better at protecting its users from infection than Microsoft.
That said, if the built-in anti-virus protection in Windows 7 is as good as some earlier reviewers suggest. the security gap could close when Microsoft's new system finally launches next month.
Which may be why Apple is hammering home the "thousands of viruses" message now.