This paper outlines a widely-held conception of illness, related to perceived changes in body temperature--'Chills' and 'Colds' on one hand, 'Fevers' on the other---in an English suburban community on the outskirts of London. The relationship between this folk model, and that of the local family physicians is analysed, to show how biomedical treatment and concepts, particularly the germ theory of disease, far from challenging the folk model, actually reinforce it. Remedies which cannot be scientifically and biomedically justified are nevertheless prescribed by the physicians to meet their patients' need to 'make sense' of biomedical treatment in terms of their folk model of illness. At the interface between physicians and patient, biomedical diagnoses and treatment are more 'negotiable' than previously realised--and this has important implications for the delivery of health care.