Infant and child physical vulnerability is demonstrated by the extremely high mortality levels in these age groups in the pre-modern West and parts of the contemporary Third World. Some children, such as females or later additions to the family, are subject to disproportionately high mortality risk. In spite of the age-old vulnerability of young children, disproportionate gains have been made in reducing their mortality in modern times. This has been a product of social and individual change, government intervention and biomedical research. These advances exhibit during crises greater resilience to reversal than might be anticipated. Rarely do infant and child mortality levels return more than a fraction of the way to the original levels. The explanation is irreversible changes to individuals and society and the persistence of health knowledge even when health facilities are paralysed.