Historically, epidemics have received limited attention except for notable events like the Black Death. The COVID-19 pandemic has renewed interest in understanding historical pandemics, particularly the role of human population movements in accelerating the spread of infectious diseases. Past outbreaks have frequently occurred in displaced populations, where the breakdown of healthcare services facilitates transmission. While wars have been suspected to exacerbate historical epidemics, a quantitative analysis was lacking. This study aims to quantify the link between conflicts, plagues, fatalities, and population evolution in pre-industrial Europe (AD 1340-1900), also considering the influence of climate deterioration.

Existing literature highlights the devastating impact of the second plague pandemic (AD 1346-1720 in Western Europe, extending to AD 1840 in other regions), beginning with the Black Death. Studies have explored the pandemic's geography, but none have quantitatively assessed the link between human behavior (conflicts) and epidemic spread. The role of climate in plague re-emergence has also been debated, with some suggesting reintroductions from Asia and others proposing European reservoirs. The spread of the plague via trade routes has also been discussed. Existing research emphasizes the impact of climate change on weakened populations through food shortages and malnutrition, increasing susceptibility to disease.

The study utilized annually resolved data from AD 1347-1840 for conflicts, plague outbreaks, fatalities, and climate. Conflict data originated from the Conflict Catalog, encompassing wars, revolts, and other displacement-causing events, summarized annually. Plague data, improved from Biraben's dataset with additions from Russia, Constantinople, and Turkey, represented recorded Y. pestis epidemics. Fatality data, from the Conflict Catalog and the US Census Bureau, were ratioed to world population and log-transformed. Climate data included temperature anomalies (European summer and Northern Hemisphere) and the Palmer Drought Severity Index. Analysis involved smoothing techniques to identify long-term trends, Mantel scalograms to test correlations, homogeneity tests to detect shifts, wavelet analysis for periodicities, and various statistical modeling for correlations and trend identification.

A strong correlation was found between conflict dynamics and the spread of plagues and fatalities between AD 1450 and 1670. The period AD 1450-1670 showed a twofold increase in conflicts compared to other periods. This period coincided with steep upsurges in plague outbreaks and a plateau in world population growth. Cross-correlations showed a positive relationship between conflicts, plagues, and fatalities (Pvalue < 0.001). Analysis showed that plague outbreaks mostly coincided with colder periods, with stronger correlations using Northern Hemisphere temperature anomalies (Pvalue < 0.001). Precipitation anomalies were not significantly associated with plague incidences. Wavelet analyses revealed significant cycles of 55 and 25 years in all time series. The study linked the increase in conflicts to the rise of plague incidences, the role of cooler temperatures in plague development, and the combined impact on fatalities. The neighbor-joining clustering highlighted that conflicts initiated the observed dynamic, which included plague and high fatalities. The siege of Caffa in AD 1343-1347, the English Civil Wars, and the Great Northern War exemplify the devastating interplay between conflict and plague transmission.

The findings indicate that warfare significantly altered Europe's human-microbe environment, promoting the spread of infectious diseases. Conflicts, through troop movements and civilian displacement, acted as vectors for plague transmission. Cooler climate conditions during the Little Ice Age further weakened populations through malnutrition and increased indoor crowding, thereby facilitating human-to-human transmission. While reintroductions from Asia and trade routes have been proposed for plague outbreaks, the study supports the hypothesis that conflicts, alongside cooler climate, played a significant role in facilitating the spread and persistence of plagues. The results emphasize the interconnectedness of war, disease, and population dynamics in shaping pre-industrial Europe's history.

This research provides the first quantitative assessment of the role of conflicts in spreading plagues in pre-industrial Europe. It demonstrates a strong correlation between conflicts, plagues, and fatalities during AD 1450-1670, coinciding with a plateau in world population. Cooler climate conditions weakened populations, further amplifying outbreaks. The study highlights the importance of robust healthcare in war-affected regions and underscores the value of historical data for calibrating models of infectious disease transmission. The findings serve as a warning about the potential consequences of overpopulation, climate change, geopolitical instability, and related factors impacting human health.

The study acknowledges limitations in the data used, particularly potential underreporting of plague events in smaller settlements. The analysis focuses on broad trends rather than individual events, and the complex interplay between various factors makes definitive causal links challenging. The study does not address all aspects of plague transmission (e.g., the specific roles of fleas and rats).