In the 1950s textbooks of nutrition made little or no mention of a relation to infection. The same was true for treatises on infectious disease. Relevant studies in experimental animals and a number of classical clinical observations were available pointing out the role of infection in precipitating nutritional disorders. However, clinicians and nutritionists did not recognize the importance of the relationship. The field and metabolic studies of the Institute of Nutrition of Central America and Panama (INCAP) in the 1950s demonstrated that malnutrition and infection in humans are generally synergistic. These studies stimulated the review of available evidence that resulted in the 1968 WHO monograph on "Interactions of Nutrition and Infection." It provided extensive evidence for the role of infections in precipitating clinical malnutrition and for the impact of malnutrition on morbidity and mortality from infection. The high frequency of diarrhea in underprivileged young children led to intensive studies in many countries of its effect on nutritional status and to recognition of the high prevalence of "weanling diarrhea." The effects of infection on nutritional status were then extensively and elegantly investigated at Fort Detrick, MD, and hormonal and cytokine mechanisms identified. The subsequent explosion in knowledge of cell-mediated immune mechanisms has led to an understanding of how malnutrition lowers this resistance. Today, recognition of the synergistic relationship between nutrition and infection influences most public health interventions to prevent malnutrition.