Inadequate dietary intake weakens the immune system and increases susceptibility to disease, infectious disease, in turn, increases nutrient requirements and further weakens the immune system. There are three underlying causes of this vicious cycle: (i) lack of availability or access to food (food insecurity), (ii) poor health mediated by poor water and sanitation and inadequate health services, and (iii), for children, poor maternal and child-care practices, including inadequate breastfeeding and nutritious complementary feeding and, for adults, poor food choices. Of course, deeper forces of social and economic underdevelopment and inequality often underpin these problems.