A method for resisting spread of unwanted code and data without scanning incoming electronic files for unwanted code and data, the method comprising the steps, performed by a computer system, includes receiving, at the computer system, an incoming electronic file containing content data encoded and arranged in accordance with a predetermined file type corresponding to a set of rules, determining a purported predetermined file type of the incoming electronic file by analysing the encoded and arranged content data, the purported predetermined file type and the associated set of rules specifying allowable content data for the purported predetermined file type, parsing the content data by dividing the content data into separate parts in accordance with a predetermined data format identified by the associated set of rules corresponding to the purported predetermined file type and determining nonconforming data in the content data by identifying content data that does not conform to the purported predetermined file format, and if the separate parts of the content data do conform to the predetermined data format, regenerating the allowable parsed content data to create a substitute regenerated electronic file in the purported predetermined file type by extracting the separate parts that do conform and putting them into the substitute regenerated electronic file.