What Mac Defender malware? The one that affected 0.1% of OS X users?
Not to mention: Mac Defender was a TROJAN. You HAD to install it yourself for it to work.
From Wikipedia: "Mac Defender (also known as Mac Protector, Mac Security,[1]Mac Guard,[2] and Mac Shield)[3] is an internet rogue security program that can be installed by unwitting users of computers running the Mac OS X operating system". The exact same thing can happen to any operating system. You can install malware YOURSELF even in OpenBSD.