Recent studies have underlined the importance of consulting different sources to trace global histories of diseases. However, due to a lack of critical editions of medical works, leprosy is poorly understood, and a wider interpretation of it as a historical phenomenon is yet to be proposed. Building on a broad critical editing and analysis of Arabic and Latin texts, this book traces a new history of leprosy moving from late antiquity to the Islamic and Latin Middle Ages, thus proving the necessity of a comparative approach to grasp its Mediterranean scope. Challenging established historical reconstructions, this study demonstrates that Arabic texts were familiar with a scientific approach to contagiousness. It also shows how, when faced with the diffusion of leprosy as an endemic disease, Latin physicians tried to solve the enigma of its nature avoiding any moral censorship. Each chapter includes the relevant texts, all related to al-Maǧūsī’s encyclopedia Kitāb al-Malakī (10th c.), in critical edition with an English translation. The book aims to contribute historians from different areas with a realistic picture of how theoretical, learned medicine considered leprosy, opening the possibility of broader research on other sources.