This article examines the historical diagnosis of bubonic plague, particularly questioning whether the Black Death (1348-1350) and other medieval European epidemics were actually caused by Yersinia pestis. The author argues that certain identification of plague is only reliable from 1894 onwards, after Yersin and Kitasato isolated the plague bacillus. Before this time, plague diagnosis is problematic because the disease relies on specific ecological conditions: the black rat (Rattus rattus), the rat flea (Xenopsylla cheopis), minimum temperatures of 21°C for flea breeding, and a rodent die-off (epizootic) preceding human infection. The Black Death spread rapidly during winter in cold conditions unsuitable for plague transmission, affected sparsely populated areas inconsistent with plague's limited diffusion patterns, and produced no historical records of rat die-offs—despite chroniclers documenting deaths of other animals. Contemporary evidence from the 1994 plague outbreak in India, where diagnostic confusion occurred despite modern serological techniques, underscores the diagnostic difficulties. The article suggests medieval high-mortality epidemics likely resulted from multiple concurrent diseases rather than bubonic plague alone, and argues plague should not be retrofitted into epidemiological scenarios that contradict its complex biology and known ecological requirements.

Key findings

• Bubonic plague can be definitively identified only from 1894 onwards when Yersinia pestis was isolated; earlier diagnosis is unreliable and uncertain.
• The Black Death exhibits epidemiological features incompatible with plague biology: rapid winter spread during unusually cold periods, diffusion through low-density rural areas, and absence of any documented rodent die-offs.
• Historical accounts of plague in the 14th-17th centuries lack essential diagnostic data including case mortality rates, rodent epizootic evidence, and clinical specificity required to confirm plague diagnosis.
• Modern diagnostic confusion in the 1994 India plague outbreak demonstrates that even with sophisticated serological techniques, plague diagnosis remains problematic in contemporary practice.
• Medieval European epidemics likely resulted from multiple concurrent diseases rather than bubonic plague alone, and plague should only be invoked when epidemiological evidence fits its documented ecological constraints.