Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_2_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148a Safari/6533.18.5)
Macs are generally immune to viruses, but Trojans are a different matter - a Trojan is simply a piece of software that claims to be innocent and useful, but is actually malware underneath. Anyone can download and install a Trojan if they're not careful - malware authors rely on people doing just that - and there are Trojans written for Mac OS X doing the rounds.
The obvious thing to do is just be very selective about what software you choose to install, and where you get it from. Pay attention to any warnings OS X tries to give you, and don't spend the whole time logged into your computer as an administrator, since that escalates the permissions that any dodgy software may be able to exploit - instead, set up a separate admin account and demote your day-to-day login to ordinary user level. You'll still be able to do anything you want, but you'll be asked by the system for your admin login anytime it's needed, which should be a handy prompt for you to think about whether program X really needs that access.