Infant-formula-feeding is inferior to breastfeeding because human milk provides specific and non-specific factors that have long-term consequences for early metabolism and the development of disease. Human milk enhances the immature immunologic system of the neonate and strengthens host defense mechanisms against infective and other foreign agents. Some mechanisms that explain active stimulation of the infant's immune system by breastfeeding are the bioactive factors in human milk such as hormones, growth factors and colony stimulating factors, as well as specific nutrients. Human milk may reduce the incidence of disease in infancy because mammalian evolution promotes a survival advantage. In addition, factors in milk promote gastrointestinal mucosal maturation, decrease the incidence of infection, alter gut microflora, and have immunomodulatory and anti-inflammatory functions. Hormones, growth factors and cytokines in human milk may modulate the development of disease. Furthermore breastfed babies have reduced exposure to foreign dietary antigen. Following the termination of breastfeeding, there is evidence of ongoing protection against illness due to protective influences on the immune system mediated via human milk. Industry continues to attempt to improve infant formula with the addition of compounds such as fatty acids, oligosaccharides, nucleotides and lactoferrin. However, human milk has such far-reaching effects on the infant's immune response that optimal development depends heavily on its provision. All mothers should be encouraged and supported to continue breastfeeding for six months and beyond in order to promote the good health of their infants.