This study will last approximately 3.5 years. Mother/infant pairs will be enrolled over a period of 18 to 24 months. During the third trimester of pregnancy, HIV infected participants will receive HIV counseling and the intrapartum/neonatal two-dose NVP prophylaxis regimen to prevent MTCT. Mothers will also be given infant feeding options counseling and information on administering the study drug to the infant. Infants who were randomly assigned to receive a placebo and older than 6 weeks of age as of 08/10/07 OR to receive NVP will continue their treatment assignment. Infants who were randomly assigned to receive a placebo and are 6 weeks of age or less as of 08/10/07 will receive open-label NVP through Day 42 of life. For all other participants, all randomized infants will receive extended NVP through 6 weeks (Day 42) of life. All eligible infants will be randomly assigned to one of two groups at Week 6 following birth. The first group will receive extended NVP treatment; the second group will receive nevirapine placebo. Randomized infants will receive the extended NVP or NVP placebo through the first 6 months of life or until cessation of breastfeeding, whichever occurs earlier. Mothers will be instructed to begin giving their infants their assigned intervention starting at Day 3 to Day 7 postpartum. All mothers and infants outside of the study will be offered the local standard of care antiretroviral (ARV) regimen for the prevention of MTCT, but these ARVs will not be provided by the study.