Salaried employment, feminism, and AIDS have provoked debate about the importance of breastfeeding and milk for child health and survival in the developed world. Immunology of the edifice is complex and, at the same time, incomplete. Nevertheless, it is useful to acquire a sense of mechanism—why the provision of human milk to human infants is adaptive behaviour. In primitive circumstances, mortal illness from gastrointestinal infections results from microbial contamination of the water supply and feeding utensils. Prior to World War II, infant mortality from acute respiratory diseases was high among infants who were not breastfed. The prevention of chronic and recurrent otitis media by breastfeeding is also a relevant facet of preventing chronic respiratory disease. In premature infant nurseries where there is a high standard of hygiene and medical care, human milk still has an important role to play in the protection of infants against life-threatening bacteremia and necrotizing enterocolitis.