Genetic sequencing allows reconstructing the specific HIV transmission chains through which the virus has diffused and evolved within a population. From the perspective of social science AIDS research, this represents a unique opportunity: such techniques (i) provide objective measurements of the (sexual) connections between members of a population, and (ii) allow new measurements of the HIV-1 diffusion process—the actual transmission of HIV and the diversification of the virus during transmission in sexual networks—whose determinants have largely been understudied. The main aim of this application is therefore to study empirically the social determinants of the transmission and diversification of HIV-1 in a sub-Saharan setting where the prevalence of HIV-1 infection is high. The specific aims of this project include: (1) Create the first large-scale complete-sexual-network study with detailed phylogenetic data in sub-Saharan Africa by complementing the already funded second wave of a unique population-based survey of sexual networks on Likoma Island (Malawi) with genetic sequencing of all identified HIV-1 cases in the study population (N 230). (2) Use the molecular-genotype data to establish chains of HIV infection, and use these reconstructed transmission patterns to (a) assess the validity of sexual network data collected during the survey; and (b) investigate the relationship between reported risk behaviors and actual transmission of HIV-1 along sexual networks. And, (3) investigate the impact of sexual network structures on the rate at which recombinant forms of the virus, dual infections as well as superinfections emerge within a population.