Early modern people understood that diseases could spread. The concept of contagion thus wielded considerable influence in the period. Yet care is needed to understand how it functioned within the larger medical framework of humoralism, a system that presented disease as a disrupted state of an individual’s constitution rather than an independent entity that moved between bodies (see Disease). Contagion theory developed significantly in the sixteenth and seventeenth centuries, but we should not assume that it meant precisely what we think it does today.
The name most closely associated with the early modern concept of contagion is that of Girolamo Fracastoro. The Italian physician’s 1546 De Contagione is often held up as the first modern treatise on the theory. However, the term and concept had much older roots. From the Latin words for "together" and "touching" (con + tangĕre), the term "contagious" can already be found describing diseases like leprosy in early fifteenth century texts. Fracastoro emphasized that "seeds" of disease (semina) could spread infection to healthy bodies, although historians have shown that such terminology can be traced all the way back to Galen.
Two diseases in particular drove forward discussions of contagion in the period: plague and syphilis. When the latter arose in the 1490s early commentators ascribed it to astrological causes. However, experience quickly showed that it passed between lovers, mothers and their children, and children and their nurses. Similarly, experience complicated the dominant explanation for plague, miasma – the idea that breathing foul air caused the disease – because people breathing the same air did not all succumb to plague. Quarantine measures in the fourteenth century show that some authorities already held that plague could spread between people and move from place to place.
The anonymous …litill boke necessary [and] beouvefull a[g]enst the pestilence (1485) can demonstrate how contagion was already influential at the dawn of the early modern period: "I say that pestilence sores be contagious because of enfect humours bodies & the reke or smoke of suche sores is venemous & corrupteth the ayer & therfore it is to flee from suche persons as be infect[ed]." This author shows nicely how contagion and miasma were not the mutually exclusive ideas that they would become for some in the nineteenth century. Far from it. Writers like this one ascribed plague to foul air but also suggested that said air could be contaminated directly by the plague-ridden body and thus infect those nearby.
Because there was not yet what scholars term an ontological notion of disease – the idea that diseases were independent entities – concepts like Fracastoro’s "seeds" or the litill boke’s "venom" played crucial intermediary roles. Early modern bodies were not infected by plague; they were infected by substances that caused plague. Contagion theory held that foreign substances entered the body and ignited pathological processes which then disrupted the equilibrium of the traditional humoral system. These substances were thus catalysts of disease, but not the disease itself. The bite of a venomous creature offered one useful mechanism to conceptualize how a tiny amount of matter could kill; hence doctors’ frequent discussions of poisons. When Fracastoro wasn’t using the botanical metaphor "seeds" to explain how something miniscule could develop into something robust, he introduced the term fomes from the Latin word for kindling, employing a combustion metaphor and giving us the term fomites which is still in use today. Such proxies for disease played a crucial role in helping contagion theory fit within the schema of traditional humoural medicine.
At times these entities were conceptualized as immaterial fumes (e.g., the litill booke’s “reke or smoke”) but increasingly pathogens were depicted as material particles. As both chemical and mechanistic models of nature and the body advanced during the Scientific Revolution, doctors increasingly portrayed bodily processes as the result of matter in motion. In his treatise De febribus (Of fevers), republished after his death in The London practice of physick (1685), Thomas Willis presented contagion in corpuscular terms, describing the motion and quality of the "Particles of the virulent Miasm." This did little to disrupt the continued blurring of miasma and contagion, and nor did it alter doctors’ primary metaphors. In the same paragraph Willis noted that when "the Blood [was] congeal’d by the Venom" it became "fill’d with the Seeds of the pestilential Contagion." Venom. Seeds. Miasm. None of these things were plague, but they caused plague and – frighteningly – could spread from one person to another: "Those that converse with the Diseased take the infection of the same Disease, so that oftentimes the same runs through whole Parishes and Cities."
From inanimate but deadly corpuscles, it was only a small step to conceive of infectious matter as alive. Advances in microscopy in the seventeenth century led to that breakthrough as some thinkers – clearly a minority in the period – began suggesting that miniscule "animalcules" were responsible for diseases. Marchamont Nedham’s Medela Medicinæ (1665) drew on early thinkers like the Jesuit polymath Athanasias Kircher to suggest the radical notion that "the Contagion of the Pestilence was… conveyed abroad, not only by the volatility of such Effluviums, Atoms, and Corpuscles, as are Inanimate, but by such also as were Animated, living Creatures, and were a sort of Invisible Worms or Vermicles."
And yet, as modern as contagion theory may look on the surface, the primacy of the individual constitution in early modern medical thought remained unchallenged. This ensured, among other things, that bodies were not assumed to react to contagious matter equally. The most common answer to the perplexing question of why only some sickened in epidemics was that certain bodies were "predisposed" to infection. "What bodies are most or least apt to be infected?" asked the physician Stephen Bradwell in A vvatch-man for the pest (1625), before explaining how pregnant women and children faced at elevated risk. However, the greatest threat came from the poor, who by their bad diet and filthy conditions "have their bodies much corrupted, and their Spirits exceedingly weakened: whereby they become (of all others) most subject to [plague]." Such ideas meant that discussions of contagion frequently elided into discussions of behavior, lifestyle and morality, as people bore responsibility for their relative levels of corporeal vulnerability. Such theories invited socially differentiated policies. For example, the London plague orders of 1630 commanded the expulsion of beggars from the city in the name of public health. Quarantine practices that allowed elites to sequester in their homes while forcing paupers into pesthouses demonstrate how early modern ideas about contagion frequently carried political undertones.
Carmichael, Ann, “Contagion Theory and Contagion Practice in 15th-Century Milan,” Renaissance Quarterly 44 (1991): 213-257.
DeLacy, Margaret, The Germ of an Idea: Contagionism, Religion, and Society in Britain, 1660–1730 (New York: Palgrave MacMillan, 2016).
Gibbs, Frederick W., “Plague, Poison and Metaphor” and “Poison, Putrefaction and Ontology of Disease” in Poison, Medicine, and Disease in Late Medieval and Early Modern Europe (London: Routledge, 2019), chapters 4 and 6.
Nutton, Vivian, “The Reception of Fracastoro’s Theory of Contagion: The Seed that Fell Among Thorns?” Osiris 6 (1990): 196-234.
Siena, Kevin, Rotten Bodies: Class and Contagion in Eighteenth-Century Britain (New Haven: Yale University Press, 2019).