Abstract
Does an infectious disease have one, singular pathogenic cause, or many interacting causes? In the discipline of medical microbiology, there is no definitive theoretical answer to this question: there, the conditions of aetiological possibility exist in a curious tension. Ever since the late 19th century, the “germ theory of disease”–“one disease, one cause”– has co-existed with a much less well known theory of “multifactorality”–“one disease, many interacting causes”. And yet, in practice, it is always a singular and never a multifactorial aetiology that emerges once the pathogenic world is brought into the field of medical perception. This paper seeks to understand why. Performing a detailed, genealogical reading of the 2003 severe acute respiratory syndrome (SARS) outbreak, it foregrounds a set of links that connect the practical diagnostic tools at work within contemporary, 21st century laboratories to the philosophical assumptions at work within late-19th century understandings of the “germ theory of disease”.