1. A computer implemented method for adjusting an aggressiveness level to use in behavior based heuristics malware detection, based on target file prevalence rates, the method comprising the steps of: determining, by a computer, a prevalence rate of a file subject to behavior based heuristics analysis to determine whether the file comprises malware, wherein the prevalence rate reflects a level of distribution of the file; adjusting, by a computer, the aggressiveness level to use in the behavior based heuristics analysis of the file, responsive to the determined prevalence rate of the file, comprising setting the aggressiveness level to a higher level for lower prevalence files and to a lower level for higher prevalence files; applying, by a computer, behavior based heuristics analysis to the file, using the aggressiveness level; and determining, by a computer, whether the file comprises malware, based on the applied behavior based heuristics analysis.