Pandemics have a major impact on households and the economy. But how common endemic viruses affect long-term population human capital and economic outcomes is not well understood. This column uses data from Denmark to explore the mechanisms and consequences of a child’s exposure to respiratory disease in early life. Younger siblings have two to three-times higher rates of hospitalisation for respiratory conditions during their first year of life compared to older siblings. The family unit plays a central role in virus transmission and birth order can influence children’s longer-term outcomes.