Systems, methods, and computer readable media for determining whether a computer file (210) has been infected with malicious code by an attacking agent. A scanning engine (205) determines whether the file (210) contains malicious code. The scanning engine (205) includes detection modules (325) for detecting particular attacking agents, and indicators of when particular attacking agents were first created. The scanning engine (205) determines a critical date for a file (210) with regards to a particular attacking agent. If the file (210) has not been changed since the critical date, the scanning engine (205) determines that the file (210) has not been infected by that attacking agent.