The fact that the semi-allogeneic conceptus can develop for 9 months, most commonly without harm, shows that protective mechanisms must exist that permit its survival. These mechanisms are so finely tuned or so locally confined that the immune rejection of the conceptus is prevented while maintaining the mother's ability to fight infection. One consequence of this immune silence may be the late onset of systemic manifestations of intrauterine infections, such as chorioamnionitis. This silence may be part of the complexity that has surrounded issues of vertical transmission of infectious disease in general and HIV in particular.