The informative commentaries pick up on various lessons from this paper, and touch on whether the potential cause of rickets identified by Snow may have contributed to its high prevalence in Britain at the time he was writing. Of interest to todays epidemiologists is also the way in which this paper relates to current debates about unifactorial versus multifactorial, or general versus specific, causes of disease and disease distribution. Snow was recognized by contemporary commentators as a convinced unifactorialist, someone who thought multifactorial causation a metaphysical abstraction assumed to account for the facts (see IJE 2002;31:92032). He focused on proximal agents of disease causation and was critical of approaches that attributed disease to general aspects on the environment. In his paper on rickets, he noted that the disease had generally been attributed to causes of a somewhat general nature, such as vitiated air, want of exercise and nourishing food, and a scrofulous taint, and went on to say that these explanations, however, did not satisfy me, as I had previously seen a good deal of practice in some of the towns in the north of England, where the over-crowding and the other evils above mentioned were as great as in London, whilst the distortion of the legs in young children was hardly present. Similarly, with respect to his work on cholera, Snow considered that a general attribution to social or environmental influences was unhelpful. As a thought experiment he suggested that if an investigation of scabiesthe itchhad been carried out a far greater association would be found between impure air and itch, than between impure air and cholera, and yet we know that impure air has no share in causing the itch. Even the most convinced anti-contagionist accepted that scabies was contagious, and Snow clearly hoped that his example would make such people think harder about the specific causes of cholera.
Debates about general or specific influences on disease and disease distribution have continued within epidemiology until today. In the current issue of the IJE, for example, we have a paper suggesting that low socio-economic position influences several important broad cause of death groups among children (p. 410), a paper demonstrating that the associations between social position and blood pressure seen in high income countries may not be present in a middle-income country (p. 400) and a report of mortality among Spanish civil servants demonstrating very different associations between employment grade and mortality according to cause of death (p. 386). Perhaps an exclusive focus on either specific aetiological factors for particular causes of death, or miasma-like general influences, are dead-ends, and models that incorporate more general distal factors and more specific proximal influences would be most informative. But are certainty is that the IJE will continue for many years to come to publish papers addressing the issue touched on by John Snow 150 years ago.
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