This article conceptualizes the various balancing acts between the pregnant woman, the placenta, the fetus, and the infectious agents. Despite the very large number of infectious insults during pregnancy, the outcome of most interactions usually is a normal newborn. We have identified only one general immune defect of the fetus and neonate: The inability to respond to polysaccharide antigens; yet, a similar defect also is found with certain polysaccharides in older children and adults. The capacity of the neonate to control severe life-threatening diseases with most infectious agents and the ability of the fetus, when infected in utero, to mount sophisticated, immune responses make it conceptually advantageous to consider the older fetus and the newborn infant as "immunodelayed" rather than as immunodeficient or immature.