This book is an exploration of illness and healing experiences in contemporary society through the veneration of saints: primarily the twin doctors Saints Cosmas and Damian. It also follows the author's personal journey from her role as a hematologist who inadvertently served as an expert witness in a miracle to her research as a historian on the origins, meaning and functions of saints. Sources include interviews with devotees in both North America and Europe.

Cosmas and Damian were martyred around the year 300 A.D. in what is now Syria. Called the "Anargyroi" (without silver) because they charged no fees, they became patrons of medicine, surgery, and pharmacy as their cult spread widely across Europe. The near eastern origin explains their popularity in Byzantine and Orthodox traditions and the concentration of their shrines in Eastern Europe, Southern Italy, and Sicily. The Medici family of Florence also viewed the "santi medici" as patrons, and their deeds were depicted by great Renaissance artists. In medical literature they are now revered as patrons of transplantation. Duffin's research focuses on how people have taken the saints with them as they moved within Italy and beyond. It also shows that their veneration is not confined to immigrant traditions, and that it fills important functions in health care and healing.

Duffin's conclusions are situated within scholarship in medicine, medical history, sociology, anthropology, and popular religion; and intersect with the current medical debate over spiritual healing. This work springs from medical history and Roman Catholic traditions; however, it extends to general observations about the behaviors of sick people and about the formal responses to individual illness from collectivities in religion, medicine, and, indeed, history.