Before Germ Theory: Imperial Epidemiology and the Atmospheric Imagination of Disease

This paper reconsiders the emergence of bacteriology by examining mid-nineteenth-century epidemiologists working across the British Empire, particularly in the Caribbean. Drawing on the archives of the London Epidemiological Society, it argues that these physicians articulated theories of disease transmission that anticipated germ theory decades before the laboratory identification of microbes. Although often labeled “anticontagionists,” figures such as J. Y. Simpson and Gavin Milroy did not simply reject contagion; rather, they struggled to describe an unseen material agent that moved through air, objects, and crowded environments, producing epidemics with identifiable spatial and temporal patterns.

Through efforts to map the historical spread of syphilis, trace outbreaks of rabies and cholera, and analyze disease in ships, barracks, and tropical port cities, these epidemiologists developed investigative practices—surveillance, comparative case collection, and environmental analysis—that framed disease as a circulatory phenomenon governed by an as-yet-unknown substance. Their language of “poisonous matter,” “fomites,” and atmospheric transmission departed both from classical miasma theory and from later microbiology, revealing a conceptual space in which physicians were, in effect, theorizing microbes without the epistemological tools to name them.

Engaging historiographical debates advanced by Margaret DeLacy and Michael Worboys, the paper contends that bacteriology should be understood less as a sudden scientific revolution than as the crystallization of questions first posed in imperial epidemiological practice. By recovering these colonial investigations—often conducted among enslaved and colonized populations—it reframes the origins of modern infectious disease theory as emerging not solely from European laboratories but from the observational sciences of empire.