Conflicts and the spread of plagues in pre-industrial Europe

One of the most devastating environmental consequences of war is the disruption of peacetime human–microbe relationships, leading to outbreaks of infectious diseases. Indirectly, conflicts also have severe health consequences due to population displacements, with a heightened risk of disease transmission. While previous research suggests that conflicts may have accentuated historical epidemics, this relationship has never been quantified. Here, we use annually resolved data to probe the link between climate, human behavior (i.e. conflicts), and the spread of plague epidemics in pre-industrial Europe (AD 1347–1840). We find that AD 1450–1670 was a particularly violent period of Europe’s history, characterized by a mean twofold increase in conflicts. This period was concurrent with steep upsurges in plague outbreaks. Cooler climate conditions during the Little Ice Age further weakened afflicted groups, making European populations less resistant to pathogens, through malnutrition and deteriorating living/sanitary conditions. Our analysis demonstrates that warfare provided a backdrop for a significant microbial opportunity in pre-industrial Europe.