In addition to macronutrients, deficiencies in certain micronutrients (vitamins and nutritionally-essential minerals) adversely affect both the innate and adaptive immune systems. Micronutrients play crucial roles in the development and expression of the immune response, and certain micronutrient deficiencies lead to immunosuppression and increased susceptibility to infection and disease. Select micronutrient deficiencies are common, especially in cigarette smokers, alcoholics, pregnant and lactating women, and the elderly. Micronutrient deficiencies often occur in the context of general malnutrition; however, common isolated micronutrient deficiencies include vitamin A, vitamin D, zinc, and iron.