The truth is, consumer-grade antivirus products can’t protect against targeted malware created by well-resourced nation-states with bulging budgets. They can protect you against run-of-the-mill malware: banking trojans, keystroke loggers and e-mail worms. But targeted attacks like these go to great lengths to avoid antivirus products on purpose. And the zero-day exploits used in these attacks are unknown to antivirus companies by definition. As far as we can tell, before releasing their malicious codes to attack victims, the attackers tested them against all of the relevant antivirus products on the market to make sure that the malware wouldn’t be detected. They have unlimited time to perfect their attacks. It’s not a fair war between the attackers and the defenders when the attackers have access to our weapons.
-- Mikko Hypponen, Chief Research Officer of F-Secure
Src: Why Antivirus Companies Like Mine Failed to Catch Flame and Stuxnet | Threat Level | Wired.com