Parasite–parasite interactions occur both within and between hosts, but the two scales are rarely considered together. Consequently, there is a gap in our ability to predict the integrated effects of interactions occurring across scales, disentangle their relative contributions to key ecological outcomes, and accurately identify the drivers of host–parasite evolutionary responses. Here, we extend the standard susceptible–infected framework of theoretical epidemiology to explicitly incorporate parasite–parasite interactions across scales. We identify where – in each step of the transmission process – such interactions may occur, at the within- or between-host levels, providing empirical examples where possible. Thus, we demonstrate how integrating the two scales provides a more complete understanding of the evolutionary ecology of multi-parasite systems and suggest future avenues of investigation.