Wow, so many "i heard" and "my uncle" and misconceptions about.
Let's run through this real quick:
OS X DOES NOT, and probably WILL NOT, have any viruses on it, ever.
Why?
Well, let's take a look:
1. OS is not built around web applications like Windows is, where it's built ONTOP of (not only DOS, but) Internet Explorer. Everything is an extension of explorer.
2. To make any system level changes, even when in an Admin account, you HAVE TO enter your name and password. Windows does not ask for this, which is why so much crap gets there.
3. If you download an application, even in the background, OS X will ask if you want to launch this app for the first time. Right there, is another security check.
4. The malware out for OS X comes in disk images and packages that pose as REAL applications, or cracks/hacks for those applications. Generally made of applescripts that, again, when trying to run, need your ADMIN password to change things. If you have a funny feeling about something, then do go through with it.
5. These items cannot self-propogate either. So, in short, your uncle, or what you've heard is flat out WRONG. It is fact that they are wrong, and until the day soeme uber hacker can figure out a way past having to get the admion's password to install it, change system level functions, slap a trojan in the Startup items folder, and get past LITTLE SNITCH (if you don't have this app, shame on you!), then we're in the clear.
edit: I hear the WOW virus ruins lives, not just hard drives.