Many news stories about the COVID-19 pandemic we are experiencing this spring have pointed out that the infection rates tend to be higher in larger US cities than in more rural areas. What has not been as fully appreciated is the fact that the influence of city size for the spread of the disease follows the same mathematical relation as the influence of city size for economic productivity. In a new paper, Social Reactor Project collaborator Luis Bettencourt and colleagues illustrate this using data for US metropolitan areas. The results suggest the same social mixing process, which accelerates as cities get larger and denser, lies behind both patterns. The Social Reactors Project addresses this same process in a historical and archaeological context. The paper is available for download here.