The study will be conducted under a placebo-controlled, double blind, randomized allocation of the study products. Study participants in each study group will be randomized to receive intravenous infusions of 3BNC117 and 10-1074 or placebo. 3BNC117 and 10-1074 will be administered at 2 dose levels (3 mg/kg and 10 mg/kg). These doses were selected based on the planned development of subcutaneous formulations for the prophylactic use of these products. The study will consist of 3 groups. Enrollment in Group 1 will begin first and participants will be enrolled at least one day apart. Eight participants will be enrolled in each group; 6 will be randomized receive the study drugs and 2 will be randomized to receive placebo. Participants in Group 1 will receive a single infusion of each antibody at 10mg/kg at Week 0. Enrollment in Group 2 will begin only after Day 28 safety data from all participants in Group 1, including ophthalmologic exams, are available. Enrollment in Group 3 will begin only after the Day 28 safety data from all participants in Group 2 is available. If > 1 grade 3 or higher adverse events deemed probably or definitely related to the study drugs occurs in a single group, the next group will not be enrolled pending safety monitoring committee (SMC) review. Participants in Groups 2 and 3 will receive 3 infusions of each antibody at Weeks 0, 8, and 16, at a dose of either 3mg/kg or 10mg/kg. The antibodies will be administered sequentially via intravenous infusion. Each antibody will be administered over 60 minutes.