The key aetiological issue of the early British public health movement is often seen to be the controversy between contagionism and anticontagionism. More fundamental, however, was a legacy from older constitutional conceptions of disease, of a distinction between exciting and predisposing causes. The centrality of predisposition was a theme in a wide range of medical writings; often predisposing causes were seen as sufficient causes for outbreaks of disease. Physicians who focused on predisposition often, though not always, took an interest in social factors that generated disease. The early public health movement associated with Edwin Chadwick represented then, not the victory of anticontagionism over contagionism, but the gratuitous rejection of a sophisticated understanding of the manifold determinants of health, in favour of a focus on a single, and hypothetical exciting cause. This article points out the inadequacies of the contagionism-anticontagionism characterization of early nineteenth-century aetiological theory, examines the concept of predispositionism as it appeared in the works of many writers, and considers in detail investigations of the Irish fever outbreak of 1817–19, where the concept of predisposition was extensively utilized, and the arguments with which the concept was rejected by Chadwick's associates Thomas Southwood Smith and Neil Arnott.