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 ([Formula: see text]), 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 ([Formula: see text]) and longer infectious period ([Formula: see text]2.8 d) than Capdepera ([Formula: see text]; [Formula: see text]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.