Both Jews and Christians in the High Middle Ages wrote descriptions of the physical dangers posed by menstruants and menstrual blood. Such fears were augmented by their readings of the translated warnings of Greek medical writers. During this period, in both Jewish and Christian circles, those literal fears became abstracted in different ways that reflected distinct theologies. Jewish mystics saw menstruants, at the onset of their flow, as possessed by the sitra aḥra, the demonic “other side,” while Christian exegetes metaphorized menstruation almost entirely, reading biblical texts as allegorical references to sinfulness. Each group applied these negative symbols to the other, but in profoundly different ways. Thirteenth-century kabbalists, who retained Judaism’s ritual purity laws, developed a rhetoric that demonized Christians by emphasizing their permanent menstrual impurity, and they used that rhetoric to attack assimilation and ward off contact between members of the two faiths. Christians, by contrast, first emphasized the metaphorical moral impurity of Jews and later came to believe that Jewish men “menstruated,” a belief that would have dire historical consequences for the Jewish communities of northern Europe.