The 1820 Mallorca plague was not a classic bubonic outbreak

The 1820 plague outbreak in eastern Mallorca—long treated as a local episode and traditionally classified as a late bubonic manifestation of the Second Plague Pandemic—offers a unique opportunity to reassess transmission dynamics in a prebacteriological context. Using a previously unexamined primary source consisting of daily counts of deaths, we combine historical analysis with a Bayesian epidemiological model to estimate the basic reproduction number ( R 0 ), the mean infectious period, and the case-fatality ratio in the towns of Son Servera and Capdepera (Mallorca, Spain). Our results show stark contrasts between the two towns: Son Servera exhibited a higher transmission potential ( R 0 ≈ 1.3 ) and longer infectious period ( ≈ 2.8 d) than Capdepera ( R 0 ≈ 1.0 ; ≈ 2.0 d). The overall case-fatality rate of 78%—together with short infectious periods and historical descriptions of rapid clinical decline—strongly suggests that pneumonic and septicemic forms contributed substantially to transmission, challenging earlier interpretations centered on bubonic plague and rodent-borne spread. Historical evidence also rules out an enzootic rat reservoir, pointing instead to human ectoparasites or direct human-to-human routes as the most likely drivers. Moreover, mortality among children increased significantly during the outbreak, indicating age-differentiated vulnerability. By integrating quantitative modeling with fine-grained archival records, this study reassesses the biological nature of one of the last major plagues in 19th-century western Europe and contributes to broader debates on plague ecology, transmission mechanisms, and the dynamics of late Second Pandemic outbreaks.