There is compelling evidence that demonstrates that severe zinc deficiency can cause substantial impairment of cellular immunity which can result in infection and even death. The effects on immunity of mild to moderate zinc deficiency are considerably less severe and may be subtle. Zinc supplementation, especially in combination with other micronutrients, can enhance immunity if an underlying deficiency is present. Supplementation with zinc as a single nutrient, especially at high doses, can have adverse effects that include interfering with copper nutriture and impairment of immune functions.