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 show in Danish administrative data that during infancy, younger siblings have two to three 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, finding lasting differential impacts of early-life respiratory disease exposure on younger siblings' earnings, educational attainment, chronic respiratory health, and mental health-related outcomes.

We investigate the effects of early-life medical treatments on the treated children and their families. We use a regression discontinuity design that exploits changes in medical treatments across the very low birth weight (VLBW) cutoff. Using administrative data from Denmark, we establish that VLBW children have better health and higher test scores. We find that these benefits spill over to other family members: mothers enjoy better mental health, and siblings have higher test scores. Maternal mental health improvements seem to be driven by better focal child health and sibling spillovers by improved interactions within the family and parental compensating behavior.