This research will evaluate how high-risk clusters and current prevention strategies affect HIV transmission patterns. There are a number of potential drivers of the epidemic, including substance use, undiagnosed infections and high number of sexual partners. However, it still remains unclear which drivers contribute to the epidemic with HIV transmission as the biological outcome. The study aims to characterize HIV clusters and correlates associated with transmission. Phylogenetic analysis will be performed to assess transmission clusters. Qualitative interviews will be conducted to describe the psychosocial and behavioral contexts associated with transmission.