This paper offers insights for marketers implementing a viral marketing (VM) strategy. We examine how various factors in the campaign drive the threshold for ‘going viral’. Using an epidemiological model of disease propagation, we develop a framework which helps marketers better understand what drives VM success. We use a simulation approach to investigate the responses to various aspects of a VM campaign that a manager may manipulate. We find strong interactions across the parameters and there appear to be significant limits to the impact of larger viral message seeding. We also examine how to balance different components of a VM campaign should a campaign fail.