The goal of the study is to formalize the concept of viral marketing (VM) as a close derivative of contagion models from epidemiology. The study examines in detail the two common mathematical models of epidemic spread and their marketing implications. The SIR and SEIAR models of infectious disease spread are examined in detail. From this analysis of the epidemiological foundations along with a review of relevant marketing literature, a marketing model of VM is developed. This study demonstrates the key elements that define viral marketing as a formal marketing concept and the distinctive mechanical features that differ from conventional marketing.