http://www.nytimes.com/2003/09/18/technology/circuits/18POGUE-EMAIL.html* Windows comes with five of its ports open; Mac OS X comes with all of them shut and locked. (Ports are back-door channels to the Internet: one for instant-messaging, one for Windows XP’s remote-control feature, and so on.) These ports are precisely what permitted viruses like Blaster to infiltrate millions of PC’s. Microsoft says that it won’t have an opportunity to close these ports until the next version of Windows, which is a couple of years away.
* When a program tries to install itself in Mac OS X or Linux, a dialog box interrupts your work and asks you permission for that installation -- in fact, requires your account password. Windows XP goes ahead and installs it, potentially without your awareness.
* Administrator accounts in Windows (and therefore viruses that exploit it) have access to all areas of the operating system. In Mac OS X, even an administrator can’t touch the files that drive the operating system itself. A Mac OS X virus (if there were such a thing) could theoretically wipe out all of your files, but wouldn’t be able to access anyone else’s stuff -- and couldn’t touch the operating system itself. Not entirely true: you can go in using line commands on the Unix portion, and there's a Net Info manager as well.
There's another point made, which I'm also not sure is absolutely true:
* No Macintosh e-mail program automatically runs scripts that come attached to incoming messages, as Microsoft Outlook does. If they mean 'no Apple' program, maybe. But I'm pretty sure you can set Nutscrape mail to view javascripts.