Abraham Caslari was a Jewish physician in Catalonia, where he and his family found refuge after the great expulsion of French Jews in 1306. In 1348, he was also one of the many physicians who found themselves treating men, women, and children during the Black Death. Rejecting the emerging consensus among physicians that the fevers were pestilential, Caslari insisted that this misdiagnosis had caused many needless deaths. The Hebrew tractate that presents his analysis and recommended treatments is one of the earliest written responses to the Black Death by a contemporary physician. Writing in the shadow of the anti-Jewish violence of the time, Caslari's exemplar is important not only for what he has to say, but because it represents a moment before genre conventions relating to Black Death tractates became fixed. This study makes the tractate available in English for the first time, accompanied by an introduction to the work and to its remarkable author.