Participation in this study will last 6 months. Infant participants and their caretakers will need to attend study visits at entry and Weeks 2, 4, 12, and 24. At entry, participants will be given LPV/r either in liquid or tablet form, depending on whether they can swallow pills. Dosing will be calculated using the WHO schedule. At all study visits, participants will undergo a physical exam and caretakers will be asked about how well the child is taking the study medications. In addition, at Weeks 4, 12, and 24, blood samples will be taken from the participant to determine health and levels of the medication in the body. The visit on Week 4 will also require pharmacokinetic testing, which means the child will need to be monitored at the hospital for 12 hours and complete six additional blood drawls. All other study visits will last 1 to 2 hours.