Thus Adelferius described one of the miraculous healings attributed to the saint Nicholas of Trani in his account of the saint's life and miracles written in the late 1090s. Demonic possession of individuals, such as the one of Rosie above, remained a central feature of demonic activity throughout the medieval period. Many incidents of possession and exorcism miracles can be found in the Bible, such as Christ casting out the unclean spirit from a man in the synagogue at Capernaum. Following these Biblical precedents, during the Middle Ages it was believed the saints, as intercessors and like the apostles, had the same exorcising ability as Jesus. The custom of bringing possessed people to the shrine of the saints was established during Late Antiquity, as their relics were understood to have the power to expel the malevolent spirit and continued into the medieval period. Therefore, exorcism miracles frequently number among the healing miracles collected at saints’ shrines. However, they are not the most commonly occurring miracle within these collections, nor are they found in all collections.

Early historians working on cases of possession in collections of miracles have repeatedly attributed these accounts to incidents of mental illness. Both Finucane and Sigal categorized the freeing of demoniacs as miraculous cures of mental affliction. Similarly early scholarly discussion on medieval madness often suggested that contemporary belief in the origin and cause of such conditions was dominated by demonological ideas. However, such generalizations can prevent us from learning more about the nuances of incidents of possession in the medieval period: how and why such states were seen as health issues which could be healed by a saint.

By examining holistically incidents of possession together with states of unreason within the context of health, this investigation builds on recent studies. The physical symptoms of possession were similar, and in some cases the same, as the symptoms of incidents classified as conditions of madness; that is illnesses of the imaginative and rational faculties that affected the cognitive functions of the brain.