Leprosy and its history provide a particularly good opportunity for studying the intellectual and scientific elaboration of the notion of contagion and its implications as expressed in social behaviour, political decisions, and ways in which cultures view or represent the world. This chapter identifies what can be taken to be the aetiological perceptions of leprosy as viewed through medical sources and practical usages, and the ways in which they touch upon other preoccupations in the Latin West from the fourth to the fifteenth centuries. Every consideration about leprosy and its postulated contagiousness and prevention is based on two recurrent and schematic views formulated during early modern times, reinforced during the Enlightenment, and then championed by Romantics and Positivists. Before the years 1230—40, perhaps earlier in southern Europe, most medical writings could offer no direct inspiration for segregative prevention. On the contrary, practitioners seem to have been ready to defend some lepers against all violent abuses or arbitrary treatment.