The researchers plan to enroll pregnant women who are infected with HIV only, with Zika virus only, with HIV and Zika, and women not infected with either virus. The study is enrolling participants in Puerto Rico and will soon recruit volunteers at sites in the continental United States and Brazil. Study participants will be provided with appropriate treatment. Women will be monitored throughout their pregnancies and for six months after giving birth. Infants will be observed for a year after birth. The study will be conducted in two parts: In the initial phase, the researchers aim to recruit roughly 200 pregnant women; if their efforts are successful, they will then try to enroll 1,800 additional pregnant women. The study is expected to run from 4 to 6 years.