Infectious Ideas is a comparative analysis of how Muslim and Christian scholars explained the transmission of disease in the pre-modern Mediterranean world. How did religious communities respond to and make sense of epidemic disease? To answer this, historian Justin K. Stearns looks at how Muslim and Christian communities conceived of contagion, focusing especially on the Iberian Peninsula in the aftermath of the Black Death. What Stearns discovers calls into question recent scholarship on Muslim and Christian reactions to the plague and leprosy. Stearns shows that rather than universally reject the concept of contagion, as most scholars have affirmed, Muslim scholars engaged in creative and rational attempts to understand it. He explores how Christian scholars used the metaphor of contagion to define proper and safe interactions with heretics, Jews, and Muslims, and how contagion itself denoted phenomena as distinct as the evil eye and the effects of corrupted air. Stearns argues that at the heart of the work of both Muslims and Christians, although their approaches differed, was a desire to protect the physical and spiritual health of their respective communities. Based on Stearns’ analysis of Muslim and Christian legal, theological, historical, and medical texts in Arabic, Medieval Castilian, and Latin, Infectious Ideas is the first book to offer a comparative discussion of concepts of contagion in the premodern Mediterranean world.