Treatment of children and infants with HIV requires modification of medication dosing according to a child's specific weight. For lopinavir/ritonavir (LPV/r), a second line treatment option that is increasingly necessary due to infant drug resistance, this dosing is often complicated and impractical in busy clinical settings. To address this, the World Health Organization (WHO) has released a simplified dosing table based on infant weight bands. This study will evaluate the absorption, safety, and tolerance of LPV/r in infants when dosed according to the new WHO guidelines.