Breastfeeding is superior to formula feeding because it has factors that have long term consequences for early metabolism and disease later in life. In this paper, the scientific evidence in support of why breast milk is beneficial for infants is summarized and the mechanisms in which breastfeeding impacts on disease are explored. Human milk may show a reduced occurrence of disease because mammalian evolution promotes survival, and because of specific factors in milk that promote active stimulation of the infant's immune system and gastrointestinal mucosal maturation decrease the incidence of infection and alter the gut microflora. Bioactive factors, including: hormones, growth factors, colony-stimulating factors and specific nutrients, may have such far-reaching effects on the infant's immune response that normal development depends heavily on its provision. All mothers should be encouraged and supported to continue breastfeeding for 6 months and beyond in order to promote the good health of their infants.