—“War of the worms” is a war between opposing computer worms, creating complex worm interactions as well as detrimental impact on infrastructure. For example, in September 2003 the Welchia worms were launched to terminate the Blaster worms and patch the vulnerable hosts. In this paper, we try to answer the following questions: How can we explain the dynamic of such phenomena with a simple mathematical model? How can one worm win this war? How do other factors such as locality preference, bandwidth, worm replication size and reaction time affect the number of infected hosts? We propose a new Worm Interaction Model (based upon and extending beyond the epidemic model) focusing on random-scan worm interactions. We also propose a new set of metrics to quantify effectiveness of one worm terminating other worm. We validate our worm interaction model using extensive ns-2 simulations. This study provides the first work to characterize and investigate worm interactions of random-scan worms...