This paper  is a milestone in the literature on infectious diseases. To put it in context, we recall that the late 19th century saw the construction of the germ theory, and its ultimate acceptance by the medical profession. The massive research effort led by Pasteur and Koch and their followers demonstrated a variety of infectious agents, catalogued their properties, and traced their pathogenesis in infected hosts. An understanding of the behaviour of infections in populations came only later, in the early 20th century, exemplified in the work of Ross on malaria , which was extrapolated to all infections in his ‘theory of happenings’ , and of Hamer on measles . But there remained a tension between those who viewed infections from the perspective of the laboratory, with its emphasis upon biological properties, and those who viewed disease from the perspective of population statistics , which lent itself to more abstract and mathematical descriptions of epidemiological patterns. Fierce battles were waged between these disciplines, as between Almroth Wright and Karl Pearson on the subject of typhoid vaccination [6, 7].