Researchers from Symantec believe that these zero-day attacks, so called because they are launched well before vendors are even aware of the vulnerabilities, are more prevalent and more potent than previously thought. Zero-day exploits are often closely guarded secrets and can be very valuable to criminals — but once details of the exploited flaws emerge in public, developers and system administrators can get to work to mitigate or halt the attacks.
Source: http://www.theregister.co.uk/2012/10/24/zero_day_study/
