In this article the immunologic, clinical, and epidemiologic evidence for altered host susceptibility to infection during pregnancy is reviewed in an attempt to determine general principles that can be applied to interpret the wide range of information available and that can be utilized for epidemiologic analysis and study design. Gestational changes in immunity are related to the maternal history of infection during pregnancy. Primary infections are distinguished from recurrent infections, and the different patterns of recurrent infection in pregnancy are defined. This classification system is then used to interpret a wide range of data. The impact of infection in pregnancy on the offspring is discussed in relation to vertical transmission and pregnancy immune status: in pregnant women the clearance, if not the incidence, of infection is similar to that in nonpregnant women; maternal susceptibility to infection alters early in gestation (at less than 12 weeks); the degree of maternal recovery from early gestational infection affects vertical transmission rates; there are few data on how patterns of infection with the major tropical parasites during pregnancy relate to vertical transmission.