UNIX virii are more common than OS 9 virii, however, there are some important considerations to bear in mind.
FreeBSD is a VERY secure base to build off.
UNIX virii tend to exploit, like arn said, apps commonly running as daemons on servers, not end user machines.
UNIX hacking is mainly with the goal of stealing data. Your vacation photos will never be as interesting to hackers as bank data, government info, etc.
Apple & many many open source programmers take security very very seriously (unlike some of our Redmond based friends).
You can make your machine inpenetrable to 99.99% of virii and hackers by not being an idiot. Use a strong password, use a firewall, close ports you don't use, don't open attachments, toss your cookies every now and again, etc. Even Windows is semi secure when you follow these rules. It is VERY unlikely that a Mac OSX machine will be specifically targetted for a root exploit, unless there is data of VALUE. Your are more likely to send you identity to a shady establishment, than have a hacker or virus get it from your machine.
Windows will be the target of most malicious teenagers who write vb virii as long as most malicious teenager code on Windows.