How so? It's a computer, running an operating system. I use what OS fits my uses at the time. For nearly everything (including most gaming, as I really only play TF2, or I use my PS3).. I'm perfectly fine in OSX. The rare time I might wanna go play something like Metro 2033 or Fallout 3.. heck, maybe I wanna go get a virus. I'll start up Windows.
Errm... It spreads via the iChat application. Not to mention in that specific example it is actually a file that gets sent, then it actually needs to get run manually by the user.
On top of that, Apple puts inside the OS (as of 10.5 I believe), a certain level of checking for known malicious files, and tells you "Yo, this file is infected. We know it is. Don't run it".
So unless you were stupid, or a Windows user (Blindly running things and pressing OK to every dialogue box).. you'd be safe.
Any other 4 year old, completely irrelevant links you'd like to post without reading the content of said link?