In this article I concentrate on the effects that ghosts have on the living people in sagas. I use examples in such Íslendingasögur as Flóamanna saga, Eyrbyggja saga, Eiríks saga rauða and Laxdæla saga. I have concentrated on two aspects of the influence of the dead on the living in these sagas, fear and physical illness, and discuss medieval Icelandic conceptions of mental disorder by examining the meanings given to fear and illness intertextually. Consequently, the article also contributes to the study of the medieval Icelandic conceptions of mind and emotion, and emphasises the problems inherent in using modern concepts in historical studies. I also give special emphasis to two diverse discourses extant in medieval Iceland: indigenous folk conceptions and foreign medical theories. I show that these views sometimes overlapped but were sometimes in conflict, which makes the definition of a single concept of ‘mental disorder’ held by medieval Icelanders difficult. In this article, I argue that for medieval Icelanders ‘mental’ was something rather physical, and, although the symptoms caused by the restless dead—fear, insanity, illness and death—would be categorized by us as mental or physical, in the sagas these were all considered bodily in nature. Moreover, I also suggest that medieval Icelanders did not make a clear distinction between emotions and physical illnesses, since emotions could be part of the illness or even its actual cause. I argue that both emotions and (physical) illness encompassed state of disequilibrium and were dependent of external agents and forces that had the power to influence the bodily balance and trigger the onset of ‘mental disorder’. Consequently, ‘mental disorder’ could be manifested also in physical illness.