The pandemic of 1918–20-commonly known as the Spanish flu-infected over a quarter of the world's population and killed over fifty million people. It is by far the greatest humanitarian disaster caused by an infectious disease in modern history. Epidemiologists and health scientists often draw on this experience to set the plausible upper bound (the 'worst case scenario') on future pandemic mortality. The purpose of this study is to piece together and analyse the scattered multi-disciplinary literature on the pandemic in order to place debates on the evolving course of the current COVID-19 crisis in historical perspective. The analysis focuses on the changing characteristics of pathogens and disease over time, the institutional factors that shaped the global spread, the demographic and socio-economic consequences, and pharmaceutical and non-pharmaceutical responses to the pandemic. This title is also available as Open Access on Cambridge Core.