Phase I will enroll pregnant women/infant pairs who are: (1) infected with HIV only; (2) infected with ZIKV only; (3) infected with HIV and ZIKV; and (4) not infected with HIV or ZIKV. Phase I will assess the feasibility of enrolling a total of 200 pregnant women/infant pairs within a year, with a target of 150 HIV-infected women, 50 HIV-uninfected women from the continental U.S. sites only, and a minimum of 20 who are co-infected with HIV and ZIKV by the end of pregnancy.