Contrary to my father's opinion, mac users need to be aware of the the insecurities in their systems:
Mac OS X has been a relatively safe environment with many users who aren’t as inured to security issues as their PC counterparts. As a result, Mac users may be especially susceptible to malware that depends on what’s known as “social engineering”—that is, taking advantage of a user’s trust — rather than any specific exploits in the operating system.
"The worst mistake Macintosh users could make would be to become complacent in their security practices deluding themselves into thinking that the Mac OS has no vulnerabilities. It does," said Neil MacDonald, vice president and distinguished analyst for information security, privacy and risk at Gartner Research
Yes, the fact is that there is malware for every operating system on the planet. OS X, nearly any flavor of *nix, and Windows (believe it or not) are all susecptible to nastyware. My prediction - with the rise of "non-power users" that are starting to buy Macs, virii will become a large and growing area of concern.
That being said, currently the system vulnerabilities for Macs (and the rest of the *nix world) are tremendously miniscule compared to Winblows, mostly because of the lack of anger towards these groups and the inherent problems in attacking systems designed around privileges and permissions. For outright protection inherent in the OS itself, I'd rather put a computer novice on a Mac instead of a Win machine any day. I can't tell you how many spyware infested machines were at the Windows-based offices I've worked at in the past - as most of these office workers tend to be middle aged and non-tech saavy.