Infectious disease risk is often influenced by host diversity, but the causes are unresolved. Changes in diversity are associated with changes in community structure, particularly during community assembly; therefore, by incorporating change over time, host community assembly may provide a framework to resolve causation. In turn, community assembly can be driven by many processes, including resource enrichment. To test the hypothesis that community assembly causally links host diversity to future disease, we experimentally manipulated host diversity and resource supply to hosts, then allowed communities to assemble for three years (surveyed 2012-2014). Initially, host diversity increased disease. Subsequently, host diversity did not directly alter disease. However, host diversity determined the trajectory of host community assembly, altering colonization by exotic host species and richness-independent host phylogenetic diversity, which together reversed the initial increase in disease. Ultimately, incorporating the temporal dimension of community assembly revealed novel mechanisms linking host diversity to future disease.