Infection and malnutrition have always been intricately linked. Malnutrition is the primary cause of immunodeficiency worldwide, and we are learning more and more about the pathogenesis of this interaction. Five infectious diseases account for more than one-half of all deaths in children aged <5 years, most of whom are undernourished. Micronutrient deficiencies have effects such as poor growth, impaired intellect, and increased mortality and susceptibility to infection. The worldwide magnitude of parasite infection is enormous. It is understood that parasites may lead to malnutrition, but the extent to which malnutrition causes increased parasite infestation is not known; thus, the conditions need to be addressed together. Nutritional deficiencies associated with pregnancy are associated with poor immune response to infection. Because this immune deficiency is partially compensated by breast-feeding, this is the single best way to protect infants from infection. Malnutrition and nutritional alterations, common complications of human immunodeficiency virus infection, include disorders of food intake, nutrient absorption, and intermediary metabolism and play a significant and independent role in morbidity and mortality. The 21st century provides new information and new challenges. With new technologies and political changes, it is hoped that a healthier, more disease-free, and better-nourished population will emerge.