It should be said that the "invulnerable OS" simply does not exist. With the Microsoft "débacle" of Windows ME and Vista (brrr!) more and more people have changed habits/mind and finally used Apple products or Linux OS. But you should be aware that Windows products are a preferred environment for viruses, rootkits et similia only because of its huge diffusion. When the balance of operating systems will put more interesting weight on Linux or OSX, virus developers will change their target.
It's already started. Here is what you find with a simple search:
2006: OSX/Leap was a Trojan-worm combo that spread through Apple's iChat instant messaging system. The infected file would pop up in a buddy's iChat window, claiming to have pictures of the then-upcoming Mac OS X Leopard. Once installed, the Trojan infects recently-opened applications with poisoned code, rendering them unusable
2007-2009: OSX/DNSChanger, OSX/RSPlug, and OSX/Jahlav were Zlob Trojans that crossed over from PCs. These Trojans posed as video codecs required to watch porn videos. Once installed, they'd alter a system's Domain Name Server (DNS) settings to divert traffic to malicious websites
2008: OSX/MacSweeper, 2009: OSX/Krowi, 2010: OSX/OpinionSpy, and so on. (from
http://securitywatch.pcmag.com/none/295168-the-ten-most-dangerous-mac-viruses)
For Linux there's a page on wikipaedia:
http://en.wikipedia.org/wiki/Linux_malwareAndroid it's a mess already.
Bottom line is: Be careful and usually don't trust marketing assertions.
