Introduction: The unique dynamics of spread of bubonic plague and the Black Death

Medieval society was a profoundly rural and agricultural society. Around 90% of Europe’s population lived in the countryside, earning a livelihood by working the land or by the proceeds from owning or possessing land. A disease that should cause tremendous mortality would have to spread with great efficiency and dynamism in the countryside.

The central tenet of modern scientific epidemiology is that ‘no matter by what method a parasite passes from host to host, an increased density of the susceptible population will facilitate its spread from infected to uninfected individuals’. However, in India, in the early phase of the plague epidemics that broke out in 1896, British physicians and epidemiological statisticians noted to their amazement that plague in India seemed to defy this self-evident epidemiological ‘truth’. This strange phenomenon was discovered by E.H. Hankin who, in 1905, the same year as the IPRC was formed, published an article with data on the correlation between plague mortality and population density in the Mumbai area, which was as clear as they were inexplicable, see Table 60.

Hankin was so intrigued by this finding that he wished to test it on historical plague data. He turned to the few data on the late-medieval plague epidemics in England that were available at the time and succeeded in producing interesting evidence to the same effect. This underpinned the validity of his enigmatic finding of inverse correlation between mortality rates in plague epidemics and population densities, but he was at a loss to explain it. Importantly, it underpinned the assumption that plague of the past and modern plague was the same disease.

Hankin’s study intrigued also the IPRC. A few years later, Major Greenwood, the epidemiologist and medical statistician of the team, reached the same ‘curious and interesting’ conclusion in his statistical studies of plague in the Punjab: ‘the rate of plague mortality tends to increase as the absolute population of the infected community diminishes’. Greenwood too gathered and analysed mortality studies of plague in England in the late-medieval and early-modern periods. Again, his conclusions confirmed Hankin’s findings. Twenty-five years later, Wu Lien-Teh reached the same conclusion with respect to plague in China: ‘the smaller the community the greater the rate of mortality’.