As I've stated several times, Mac OS X's vulnerability to malware is limited by the very small number of malware programs that can run on a Mac. Although more could certainly appear, the small number of malicious programs, combined with Mac OS X's inherent security features, make it reasonably safe from most malware even if you don't change your habits (as I discuss in the next section) or install anti-malware programs (covered later in this chapter).
Nevertheless, the situation of each Mac and each Mac user is a bit different from the next. Although it's true on the one hand that Macs are resistant to malware and on the other hand that exceptionally virulent malware could find its way onto nearly any Mac, certain factors increase or decrease your risk. You can roughly assess your individual vulnerability by taking stock of how many risky activities you undertake and how often.
Some of the things that can put your Mac at greater risk from malware are the following:
Gullibility. I mean no disrespect to innocent and trusting individuals, but if you're trying to protect your security and privacy, credulity does one a disservice. If you're prone to believe most of what you read, even if you don't know (and have a good reason to trust) the source, you're more likely than most people to unwittingly participate in the spread of malware.
Visiting unsavory sites. You know the sorts of sites I'm talking about: those that traffic in porn, online gambling, ...