Infections, no matter how mild, have adverse effects on nutritional status. The significance of these effects depends on the previous nutritional status of the individual, the nature and duration of the infection, and the diet during the recovery period. Conversely, almost any nutrient deficiency, if sufficiently severe, will impair resistance to infection. Iron deficiency and protein-energy malnutrition, both highly prevalent, have the greatest public health importance in this regard. Remarkable advances in immunology of recent decades have increased insights into the mechanisms responsible for the effects of infection. These include impaired antibody formation; loss of delayed cutaneous hypersensitivity; reduced immunoglobulin concentrations; decreased thymic and splenic lymphocytes; reduced complement formation, secretory immunoglobulin A, and interferon; and lower T cells and T cells subsets (helper, suppressor-cytotoxic, and natural killer cells) and interleukin 2 receptors. The effects observed with single or multiple nutrient deficiencies are due to some combination of these responses. In general, cell-mediated and nonspecific immunity are more sensitive than humoral immunity.