This thesis explores the literary topos in which heresy is defined in terms of disease, focusing particular attention on the reign of the Byzantine emperor Alexios I Komnenos (1081-1118). By examining the portrayals of two heretics – the philosopher Ioannes Italos and the dualist Bogomil heresiarch Basileios – in a body of interrelated source material, conclusions are drawn related to the contemporary thought-world, which influenced the authors, their works and their understanding of the heterodox threat. This, in turn, is used to gain insight into the contemporary dynamics of imperial propaganda and power. There are four main chapters, the first of which discusses the methodological approach adopted throughout this study. This section treats various questions related to the problems inherent in heresy scholarship, such as the ever-changing definition of ‘heresy' and the use of source material that is fundamentally antagonistic towards the heretical subject. The second chapter traces the transmission of the focal topos, ‘heresy as disease', within heresiology from its origins in the fourth-century Panarion of the bishop Epiphanios of Salamis up to the twelfth century, where it is found used prevalently by the court of Alexios I. Chapter three then offers a detailed analysis of the primary sources that are employed in the case studies of Italos and Basileios: Anna Komnene's Alexias, Euthymios Zygabenos's Panoplia Dogmatike, the Synodikon of Orthodoxy and trial proceedings preserved from the synodal examination of Italos. The final chapter explores the surviving presentations of both men – their depictions as ‘outsiders' and the specific association developed between their teachings and disease – within the context of the newly emerging and insecure Komnenian dynasty. ‘Heresy as disease' is found to transmit an ideological framework, allowing Alexios to reinforce his unstable position by capitalising on the image of the great Orthodox doctor, providing a cure for the common soul.