A negative association between interval length and infant and child mortality has been consistently identified in modern developing countries. The reasons for this association are unclear, however. Leading hypotheses explain these differences as a result of sibling competition, maternal depletion, infection transmission, or unobserved maternal factors, but none has received overwhelming support. Using data from Stockholm between 1878 and 1926, this study contributes to the body of research by identifying trends in the relationship over time, controlling for unobserved maternal heterogeneity, and exploiting sibling deaths to better understand the mechanisms at work. Results show the association disappeared over time as infectious disease mortality fell and that deaths of previous siblings during the postnatal period disproportionately tended to increase the risk of dying among index children born after short intervals. These findings strongly suggest the relationship is related to the transmission of disease between closely spaced siblings.