For medieval Latin Christendom, authoritative texts such as the Bible and the writings of the Fathers of the Church provided a skeleton that gave form to Christian perceptions of Jews and Judaism. Eye-witness testimony, hearsay, reports of converts from Judaism, and the testimony of dreams, visions, and miraculous events helped fill in the body with concrete detail. In this newest work, renowned author and scholar Irven Resnick explores the additional support drawn from medieval science. Resnick presents a captivating study of long-held medieval scientific theories that predisposed Jews to certain types of offensive behavior or even to communicate certain illnesses and disease. By arguing for a Jewish ""nature"" dictated by specific physical characteristics, medieval scientific authorities contributed to growing fears of a Jewish threat.

Through the use of several illustrations from illuminated manuscripts and other media, Resnick engages readers in a discussion of the later medieval notion of Jewish difference. Externally, these differences were evidenced by marks that physically distinguished Jews—and especially Jewish males—from their Christian neighbors, for example the mark of circumcision. Internally, their melancholy humoral complexion, further weakened by the Jews' dietary restrictions, was thought to dictate their temperament and sexual mores, and to incline them toward leprosy, bleeding hemorrhoids, and other infirmities. These differences were viewed by some as ineradicable, even following religious conversion; or, at best, erasable with only the greatest difficulty over several generations.