The impact of the global human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) epidemic is most severe in sub-Saharan African countries already affected by undernutrition and food insecurity. There is a two-way relationship between HIV and undernutrition and food insecurity, which is mainly synergistic and operating at different levels. HIV infection increases energy and nutrient requirements, yet it reduces food security. The result is nutritional deficiencies, which increase progression of HIV infection. Both undernutrition and food insecurity may also lead to increased risk of transmission. Nutritional intake and status may affect metabolism of antiretroviral drugs, some of which may affect body composition, and increase risk of the metabolic syndrome. In addition, HIV is transmitted through breastfeeding, causing a serious infant feeding dilemma for which the current recommendation is to provide all pregnant and lactating women with antiretroviral therapy.