Preschool-aged children get sick frequently and spread disease to other family members. Despite the universality of this experience, there is limited causal evidence on the magnitudes and consequences of these externalities, especially for infant siblings with developing immune systems and brains. We use Danish administrative data to document that, before age one, younger siblings have 2-3 times higher hospitalization rates for respiratory conditions than older siblings. We combine birth order and within-municipality variation in respiratory disease prevalence among young children, and find lasting differential impacts of early-life respiratory disease exposure on younger siblings’ earnings, educational attainment, chronic respiratory health and mental health-related outcomes.