Viruses:
Mac Virus FAQs: Do You Really Need Mac Antivirus Software?
"2. Why are Macs less prone to infection?
Unlike Windows, Mac OS X applications don't share a common registry. Mac OS X applications use individual preference files, thus the types of global configuration changes which enable so much of Windows malware is simply not as feasible on a Mac. Further, on Mac OS X, the Web browser is not integrated into the operating system - it's a completely separate/independent application. This isolates the browser - a common infection point for Windows users - and helps insulate the Mac OS from external attack. Root access is needed in order for malware to interact with other programs (i.e. steal passwords, intercept transmissions, etc.) By default, root access on a Mac is not enabled.
3. Are there any real Mac viruses out there?
Some try to answer this question literally, based on the strict definition of 'virus' - i.e. malicious software that infects other files. But the term 'virus' is used much more loosely these days and in that context refers to malicious software in general (or what the industry terms 'malware'). The answer also depends on the version of the Mac operating system (OS) in question. While Windows tends to be essentially the same "under the hood", the various flavors of the Macintosh OS vary widely. Thus the answer to the question is Yes, there are real Mac viruses out there. But whether you are vulnerable or not depends on the OS. As for malware in general, it's an even stronger Yes."