Infectious Ideas is an impressive comparative study of responses to the plague amongst Muslim and Christian communities in the premodern Western Mediterranean. Justin Stearns masterfully navigates through Muslim and Christian literature as diverse as plague treatises, sermons, fatwas, scriptural exegesis, as well as theological and medical texts written over the course of a thousand years in order to highlight the variation that existed within these communities’ understanding of the plague and its transmission. By doing so, Stearns challenges earlier studies that assumed that differences in responses between the communities were due to essential civilizational characteristics. Such studies accepted, and further reinforced, the widespread notion that Islamic societies had turned away from reason and gone into “decline” in the post-1200 period, reflected in their “typical” fatalistic attitude towards the plague and rejection of contagion. At the same time, Christian European societies were seen as being more scientific during this period, reflected in their acceptance of contagion and institution of quarantine laws. Infectious Ideas rejects both this traditional “decline” theory, and this flattening of the diversity of opinions within each society. Consequently, it provides us with a very rich account of the complex ways in which tradition, sociopolitical realities, reason, and empirical observations were negotiated by both Christian and Muslim scholars in the premodern Mediterranean.

The book's real strength lies in its deployment of a number of key methodological developments of the last two decades. The work of Andrew Cunningham and others on 19th-century transformations in disease etiology due to the “laboratory revolution” provides Stearns with the tools to understand premodern rejections of contagion as measured, rational responses to contemporary observations and medical understanding of the plague, rather than the blind, irrational whims of a conservative, religious elite. Similarly, the book follows current trends in the study of Mediterranean societies, based on the work of Peregrine Horden and Nicholas Purcell, among others, that seek to embrace the diversity and complexity of these societies while undertaking comparative histories rather than resorting to unchanging, essential characteristics. Finally, Stearns builds upon recent work that challenges the sharp division between “science” and “religion” in premodern theistic societies, in order to show the rich interconnections between the two, whether that be the incorporation of medical considerations in theological texts and fatwas, or the deployment of religious and metaphorical understandings of contagion in medical texts.

The book is divided into six chapters, in addition to an introduction, conclusion, and two appendices. Four of those chapters deal with societies under Muslim rule, and two with those under Christian rule. The slight imbalance here is reflective of the fact that Stearns’ primary concern is to demonstrate the richness and thoughtfulness of the Islamic discourses on plague and contagion, which have been historically dismissed as being irrational and antiempirical. The back-and-forth between chapters on Muslim and Christian discourses allows Stearns to illustrate the similarities in the range of responses within each tradition, and their similar struggles in understanding disease transmission prior to the laboratory. The chapters thus highlight his comparative methodology, which accepts the reality of differences between traditions based on differences in their foundational texts, yet emphasizes that as subsequent scholars rework the tradition, often in confrontation, competition, and/or dialogue with other groups of scholars, they may come to express views similar to views found in other faith traditions.

For example, Stearns shows that the Muslim discourse was shaped by (1) its concern for God's unity (against polytheism) and omnipotence and (2) an early encounter of the Prophet and his companions with the plague, leprosy, and other diseases. The former gave rise to hadith that rejected contagion, for the communicability of diseases of their own accord was seen as challenging God's omnipotence. The latter, on the other hand, gave rise to hadith and reports of the Prophet's companions that suggested the possibility of contagion. Muslim scholars thus had to reconcile this apparent contradiction within their tradition, and made specific choices based on contemporaneous debates over the validity of hadith (Chapter 1) as well as debates between theologians and philosophers over the nature of the world, its eternality, and causation (Chapter 3). This earlier, rich discourse was further enlivened by Western Mediterranean Muslim scholars in the aftermath of the plague, as they sought to take into consideration the existing empirical evidence and the psychological and spiritual ramifications of the plague for their communities (Chapters 5 and 6). In the end, the premodern Muslim discourse came to include a diversity of views arguing for and against contagion and offering recommendations for both fleeing the plague and staying back to care for the sick.

The Christian discourse, on the other hand, emerged within a Hellenistic concern over the contagious nature of heresies. It thus developed within a context that regularly employed the metaphor of contagion to place restrictions on fraternizing with heretics—a fear heightened by the Muslim conquest of Iberia. The metaphor was only secondarily medicalized and applied to disease transmission (Chapter 2), but in a way that could incorporate discussions of the evil eye just as easily as those of the plague under the rubric of contagion in medical texts (Chapter 4). Thus, whereas earlier scholars had characterized Muslim and Christian responses in essentialist terms, referring to the former as social and fatalistic, and the latter as individual and rational, Stearns shows how both groups of scholars were rationally confronting the realities of disease transmission, within the constraints of prelaboratory empirical observations and a rich, received tradition.

There are perhaps two minor weaknesses in the book that are worth highlighting here, if only to stimulate more work in the area. First, the book does not entertain the possibility that the medical understanding of plague and disease transmission may have evolved over time, and thus overlooks medical texts and commentaries produced during the later period. An analysis of such texts would have added more insight into Muslim responses to the plague. In particular, a closer examination of the evolution of the definition and employment of terms such as wabāʾ, ṭāʿūn, iʿdāʾ, and al-maraḍ al-wāfid would shed further light on the intersections among medical, juridical, and theological discourses, and thus would only help strengthen the book's main thesis. Second, the discussion of the Christian communities ends a bit abruptly. A short section or chapter that illustrated how the new focus on the metaphorical entrenchment of contagion in Christian societies can help us understand some peculiarities in Christian responses to the plague in Iberia would have been helpful. Nonetheless, these are minor points that themselves can only be noticed because Stearns has done an impressive job of showing why our earlier essentializing of plague responses in Muslim and Christian communities was so wrong.