The optimal level for any individual’s nutrient status is determined by a complex web of interacting parameters including their genetic background, environmental exposures, and interactions with other nutrients. In developing countries, and for certain nutrients (especially iron), host-pathogen competition for the nutrient adds an extra layer of complexity. Attempts to increase iron status conflict with the likelihood that excess iron might precipitate infections. The figure indicates that the optimum level (indicated by the nadir in the curve) varies according to host genotype (e.g., resistance factors involved in nutrient handling) and the genotypes of various pathogens (e.g., mechanisms of iron sequestration and consequent responses). By removing pathogen threats, as has largely been achieved in developed countries, it becomes possible to safely increase the optimum level of nutrient status with a view to improving cognitive and developmental outcomes.