The current Covid-19 pandemic has underscored the need to thoroughly revisit our conceptions of managing and developing urban systems and to make them more resilient to epidemics. For example, it fundamentally questions the long-time goal of continually increasing urban mobility. However, the relevance of resilient strategies depends heavily on our understanding of the dynamics of epidemics. In this communication, we show both theoretically and empirically that scaling is a fundamental characteristic that is not taken into account by classical epidemic models, whereas it easily results from cascade models aboth in phases of growth and decline. We highlight the resulting consequences for data analysis and simulation. They are illustrated using the Johns Hopkins University Center for Systems Science and Engineering database.