A method, system, computer program product and/or a computer readable medium of instructions for detecting one or more entities which are able to reinfect a processing system with malicious software. The method includes: monitoring, in the processing system, activity indicative of the malicious software reinfecting the processing system; in response to detecting the activity, storing a record of the activity and one or more entities associated with the activity; determining if the malicious software has reinfected the processing system; and in response to determining that the malicious software has reinfected the processing system, analysing the record to detect the one or more entities which were associated with the activity that caused and/or assisted in reinfecting the processing system with the malicious software. There is also disclosed a method, system, computer program product and/or a computer readable medium of instructions for detecting a variant of malicious software in a processing system.