In his personal notebooks, the little known Bohemian physician Georg Handsch (1529–c. 1578) recorded, among other things, hundreds of vernacular phrases and expressions he and other physicians used in their oral interaction with patients and families. Based primarily on this extraordinary source, this paper traces the terms, concepts and images to which sixteenth-century physicians resorted when they explained the nature of a patient’s disease and justified their treatment. At the bedside and in the consultation room, Handsch and his fellow physicians attributed most diseases to a local accumulation of impure, putrid or otherwise pathological humours. The latter were commonly said to result, in turn, from an insufficient concoction and assimilation of food and drink in the stomach and the liver or from an obstruction of the humoral flow inside the body and across its borders. By contrast, other notions and explanatory models, which had a prominent place in contemporary learned medical writing, hardly played a role at all in the physicians’ oral communication. Specific disease terms were rarely used, a mere imbalance of the four natural humours in the body was almost never inculpated, and the patient’s personal life-style and other non-naturals did not attract much attention either. These striking differences between the ways in which physicians explained the patients’ diseases in their daily practice and the explanatory models we find in contemporary textbooks, are attributed, above all, to the physicians’ precarious situation in the early modern medical marketplace. Since dissatisfied patients were quick to turn to another healer, physicians had to explain the disease and justify their treatment in a manner that was comprehensible to ordinary lay people and in line with their expectations and beliefs, which, at the time, revolved almost entirely around notions of impurity and evacuation.