The neonate is deficient in the main antibody that protects mucosal membranes, the secretory IgA. While developing this immune system the breast-fed baby is provided with 0.25-0.5 grams per day of secretory IgA antibodies via the milk. These antibodies which function in concert with other defense factors in milk such as lactoferrin are directed against a number of micro-organisms that threaten the neonate. Recent studies suggest that it may be possible by vaccination of the mother to increase the immunity provided the breast-fed infant via the milk secretory IgA antibodies. Breast-feeding results in a lower frequency of infections in the infant, not only developing countries, but also in societies like Canada and USA. In developing countries the most dangerous period of a child's life begins with weaning when the protection of the breast milk vanishes and often heavily contaminated food is introduced. The large number of infections, especially diarrhea, that follow may be a major factor impairing growth and development with accompanying undernutrition. Utilization of available nutrients is much improved if these infections can be prevented.