Project Summary/Abstract Social networks (i.e. social, sexual and substance use contacts) impact the spread of HIV and sexually transmitted infections (STIs) and can be used as a tool for public health prevention efforts. PWID are embedded in social networks that may influence their HIV risk as well as their risk reduction behaviors and engagement with the HIV care cascade. Clinical data and diagnoses of infectious diseases can be used to inform the networks. In the U.S., STI incidence rates are the highest they have been in over 25 years. STIs can increase the risk of HIV acquisition and among those who are living with HIV, STIs can increase the risk of onward HIV transmission. In addition, STI diagnoses may predict future incidence of HIV. Studies are needed to better understand the prevalence of STIs among PWID and the potential impact of STI screening on risk of HIV among PWID. Sex and drug tourism from the U.S. to Mexico has played an important role in HIV spread in the border region, as these factors drive bidirectional cross-border mobility. Recent work provides evidence of extensive HIV viral exchange between San Diego, CA and Tijuana, Mexico, with the direction recently reversing to viral migration now being predominantly from Tijuana toward San Diego. In Aim 1, we will assess the prevalence of STIs (syphilis, Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) infections) in San Diego among PWID living with and at risk for HIV and will assess the potential additional risk of those infections among sex and drug tourists. In Aim 2 we will identify where STI and HIV infections are within a social network of PWID and assess whether they can be used to understand the HIV risk context. We will leverage an ongoing cohort study among PWID in San Diego, California, La Frontera (R01DA049644, PI: Strathdee) which uses field based mobile testing and survey methods. The proposed study aims meet the A-START goals of facilitating the entry of an early career investigator into the intersection of drug abuse and HIV/AIDS research and will provide pilot data for a future longitudinal R01 proposal aimed at using social and transmission network in combination to predict HIV and prioritize prevention activities. The results of the proposed study will additionally provide evidence for public health researchers and practitioners on best practices for STI screening among PWID and potential HIV prevention. This study addresses the Trans-NIH plan of the OAR, by conducting multidisciplinary research to inform implementation of timely interventions to reduce incidence of HIV.