Pregnant women with HIV infection entering this study will be assigned to Cohort 2, as all infants in Cohort 1 have completed the 6 to 8 week study visit and all Cohort 1 data have been reviewed. Mothers in Cohort 2 will receive a single dose of 900 mg of TDF combined with 600 mg emtricitabine, along with standard ZDV prophylaxis and/or other antiretrovirals prescribed by her physician. Infants will receive a single dose of TDF at 4 mg/kg combined with 3 mg/kg emtricitabine as soon as possible after delivery and within 6 hours of age as well as the standard 6 weeks of oral ZDV prophylaxis after birth. Blood samples from mothers and infants will be taken as for Cohort 1.