The Germ of an Idea: Contagionism, Religion, and Society in Britain, 1660–1730 by Margaret DeLacy (review)

The bacteriologist and historian Charles-Edward Amory Winslow once posed the question: why had no serious thinker by the year 1700 arrived at the modern germ theory of disease? Given the advances in microscopy, generations of attacks on Galenic orthodoxy, and the experience of smallpox, plague, and syphilis, did no one wonder if specific diseases might be caused by specific types of minute, living organisms? In returning to the decades around 1700, Margaret DeLacy's The Germ of an Idea arrives at several conclusions: first, that some writers did argue that certain diseases are contagious, speculating that worms, insects, or animalcule are the true agents of disease; second, that the proponents of contagion and of the idea of a living agent of contagion remained marginal figures, never able to mount an effective challenge to the dominant Galenists and neo-Galenists of the Royal College of Physicians; and third, that the experiences of smallpox inoculation and the terror sparked by the 1720 plague of Marseilles did at least confer an enduring credibility on the notion of contagion.

Very large conceptual gaps separate modern germ theory from the standard views of the Restoration medical establishment. We know that many diseases are caused by specific microorganisms which can spread from person to person or via animal vectors and that they cause predictable sets of symptoms. This is the ontological view of disease. The Galenists and neo-Galenists who dominated Oxford, Cambridge, and the Royal College of Physicians subscribed to a rival paradigm according to which sickness occurs as a result of the interaction between an individual and his or her environment. Advocates of this physiological approach usually insisted that the particular constitution of the patient determines the kind of sickness that he or she develops when exposed to disruptive influences. DeLacy shows how a small number of British medical writers, between the 1660s and 1730, deviated from this standard model. Several authors, including long-remembered figures like Hans Sloane and Richard Mead, argued for the contagiousness of major killers like consumption, plague, smallpox, syphilis, and leprosy. Others alleged that tiny organisms – insects, worms, or animalcule – cause sicknesses. Writers tended to adopt, at most, one or two elements of germ theory. For instance, they could (like Marchamont Nedham) blame disease on living organisms but claim that they are so omnipresent in the air that there is no place for direct person-to-person transmission. Or, they might accept (like Robert Hooke) that people can infect others but deny that the infective material is living. Only a few, like the obscure London physician Benjamin Marten, put the pieces together to create something akin to modern germ theory. The Germ of an Idea does an excellent job of conveying the diversity of ideas about both contagion and the nature of the hypothesized sickness-inducing agent.

In the course of describing the speculations of the few writers who anticipated germ theory, DeLacy also usefully describes the insuperable obstacles to it becoming a new orthodoxy. Some of these reasons had to do with the quality of the evidence. These included: a complex epidemiological picture that did not always appear to lend support to contagionism, the failure of contemporary microscopists to identify tiny organisms that were associated with specific diseases, and the difficulty physicians had in agreeing that outbreaks of disease in different locations were actually the same disorder. But The Germ of an Idea makes clear that social, religious, and political factors bulked every bit as large. Most members of the Royal College of Physicians upheld the notion that medical wisdom is to be found in books rather than derived from empirical inquiry and they were accordingly skeptical of ideas about which the ancients had been relatively quiet. Those who did embrace an ontological theory of disease, in contrast, were socially and politically disposed to challenge authority. Some were Dissenters who had been...