The presence of power-law connectivity distributions and small-world characteristics in current unstructured overlay networks, so useful to speed up the communication process, ironically, also exposes some fatal topological weaknesses, e.g., being extremely vulnerable under intentional targeted intrusions, which seriously reduces their intrusion survivability. As a remedy, we in this paper propose a novel generalized and practical analytical formulation called
Economic Defense Model
, to characterize the intrusion spreading in these networks and provide guidelines for controlling the epidemic outbreaks. Based on (but much different from) currently existing methods, our model focuses on two key concepts of efficiency and cost, by giving deep insight into the role of topological properties, like the scale free behaviors, the small-world-like phenomena, the statistical significance of both nodes and links during dynamic topology evolution over time. Moreover, we propose a novel economic defense strategy and then perform a case study to examine how efficiency and economy principles combine up to shape the epidemics and immunization in these overlays.