There will be eight study visits for infants, which will occur within 24 hours of birth; on Day 3; between Days 5 and 7; at Weeks 6 and 12; and at Months 6, 9, and 12. Medical history, a physical exam, and blood collection will occur at all visits. Infants will have x-rays to assess bone health at Day 3 and Month 3, except in Cohort 4, which will not include x-rays of infants. Infants of Cohort 1 will have blood collection for PK studies at birth and four times after birth. Infants of Cohorts 2 and 3 will undergo blood collections for PK studies at birth, Day 3, and Day 5. Blood collection at these visits will occur before receiving TDF and 2 and 10 hours after receiving TDF. At birth, an additional collection will occur 18 to 24 hours after receiving TDF, and on Day 5, two additional collections will occur--at 18 to 24 hours and at 36 to 48 hours after receiving TDF. Infants of Cohort 4 will have blood collection for PK studies at birth and after their fourth and seventh doses of TDF.