Apple implements internally an encryption mechanism to protect some of their own executable like "Dock.app" or "Finder.app". This encryption can be applied to malware as well. If one does, anti-malware solutions can no longer detect the malware because of the encryption, but OS X has no problem loading such malware.
This same protection mechanism can be used on existing malwares that are already detected by Anti-malware products, to make them completely undetectable. Those same anti-malware products can no longer detect the malware because they don't understand it's encrypted.
The events of recent years have led many users to question just how secure Mac really is.