This review examines the implications of the fetal and infant origins of disease hypothesis for developing countries. Additionally, the paper looks beyond the chronic diseases of affluence involving susceptibility to infectious diseases. It is estimated that more than 90% of the 25 million low-birth-weight babies born each year are born in developing countries. Statistical analysis shows that poor maternal diet is the key factor in small birth size. In combination with low birth weight, fetal and infant origins of diseases are of particular importance. This aspect has been explored in several studies, which support the main hypothesis and concept of the thrifty phenotype. In addition, new evidence has emerged that nutritional programming during intrauterine life may affect the development of the immune system of the fetus. Although this finding has emerged in a unique context, which combines a natural situation with long-term demographic records, other evidences support the strong interdependence of nutrition and immune system functioning. Such discovery may be of importance to any growth-retarded fetus and may have implications that extend beyond the classic infectious diseases to cancers, autoimmune disorders, and other morbidities.