Why do certain cultural items capture persistent collective interest while others languish? This research integrates psychological and sociological perspectives to provide deeper insight into social epidemics. First, we develop a psychologically plausible individuallevel model of social transmission behavior. We then situate this model in a social network and perform a series of simulations where we vary different item- and networkrelated characteristics in an experimental setting. The results (1) demonstrate how item and network characteristics combine to drive persistent collective enthusiasm and (2) shed light on the underlying mechanisms through which such social epidemics occur. Interest in most items or products naturally decays over time, so item characteristics (e.g., talkability) and the network positions of early consumers are critical for bolstering consumer enthusiasm. Importantly, however, they do so via different mechanisms, determining how frequently, and with what level of enthusiasm, items are discussed.