In this paper I present the genesis of R0 in demography, ecology and epidemiology, from embryo to its current adult form. I argue on why it has taken so long for the concept to mature in epidemiology when there were ample opportunities for cross-fertilisation from demography and ecology from where it reached adulthood fifty years earlier. Today, R0 is a more fully developed adult in epidemiology than in demography. In the final section I give an algorithm for its calculation in heterogeneous populations.