A study reveals new clues to Europe's last major plague outbreak

In May 1820, an outbreak of plague began in the eastern part of the island of Mallorca which had traditionally been classified as a late episode of bubonic plague. Now, the study 'The 1820 Mallorca Plague Was Not a Classic Bubonic Outbreak', published in the prestigious scientific journal PNAS and carried out by professors Pere Puig, from the Autonomous University of Barcelona (UAB) and the Center for Mathematical Research (CRM), and Joana Pujadas-Mora, from the Open University of Catalonia (UOC) and ICREA academic researcher, analyzes this outbreak and reviews how the plague was transmitted in a context before the development of bacteriology.

The research shows that in that episode the mortality reached 78% and according to historical documents, the patients got worse very quickly. These characteristics suggest that the pneumonic and septicemic forms of the plague played a major role in the outbreak, contrary to previous interpretations, which attributed it primarily to bubonic plague transmitted by flea bites associated with rodents. Historical data also rule out that rats acted as a reservoir of the disease and point to human ectoparasites, such as fleas and lice, or direct contagion between people as the most likely routes of spread in the 1820 episode.

According to research, the 1820 outbreak in Mallorca probably arrived from Tangier through the crew of a ship, already infected, who disembarked clandestinely in the area of ​​Son Servera, in the east of the island. The town had no harbor and goods were landed in small rowboats, making it unlikely that infected rats would reach land. The most likely, according to the study, is that infected fleas from the ship bit Son Servera residents, who subsequently fell ill. Another possibility is that the disease arrived through close contact with infected crew members. From this first focus, the plague could have spread rapidly through human ectoparasites, such as fleas or lice, and also through direct transmission between people of the septicemic and pneumonic forms.

The importance of confinement

However, the epidemic did not spread massively thanks to the strict isolation measures that were implemented once the outbreak was confirmed. The towns of Artà, Son Servera and Capdepera were cordoned off and also, later, Manacor. These measures which entailed a strong military presence and reduced trade and movement between towns demonstrate the awareness on the part of the authorities that the disease was contagious.

In total, more than 2,400 of the area's 7,500 inhabitants died during this episode, which is known as the last major plague outbreak in 19th-century Europe.

This research questions traditional interpretations of the disappearance of the plague in Western Europe and highlights the role of human behavior, mobility and ectoparasites in the evolution of epidemics.

The study is based on a previously unexamined daily historical record that systematically recorded the number of sick, dead, convalescent and recovered patients throughout the outbreak. By combining this historical documentation with advanced quantitative methods, the research provides new information about the nature of the outbreak and provides a better understanding of how plague epidemics developed before the bacteria responsible were known. Thus, it contributes to connecting historical evidence with current knowledge about the transmission of diseases.

This work demonstrates the value of combining mathematical and statistical modeling with archival sources to reconstruct past disease ecology and improve our understanding of pathogen transmission in historical populations.

Reference article: Puig, P., & Pujadas-Mora, J. M. (2026). The 1820 Majorca plague was not a classic bubonic outbreak.Proceedings of the National Academy of Scienceshttps://doi.org/10.1073/pnas.2536892123