To contain the HIV epidemic in Kenya and other parts of sub-Saharan Africa, innovative strategies for identifying and treating HIV-infected persons who inject drugs (PWID) are critically important. It is also essential that factors contributing to onward transmission are better understood if prevention and treatment programs are to be brought to scale for key affected populations. The study will be co-led by Dr. Carey Farquhar and Dr. Joshua Herbeck and is a collaborative effort between the University of Washington, Kenyatta National Hospital, the University of Kwa-Zulu-Natal, Kenya Ministry of Health and a local NGO, Support for Addiction Prevention and Treatment in Africa (SAPTA). The study will determine whether assisting HIV-infected PWID to notify their partners can be a successful strategy for HIV testing and linkage to care, and whether assisted partner notification services (APS) can be leveraged to identify individuals at high risk for hepatitis C (HCV). The primary objective is to determine how many needle-sharing and sexual partners per HIV-infected PWID accessing HIV testing services can be tested, diagnosed HIV- or HCV-positive, linked to appropriate services, and engaged in care at 3 months post-testing. To achieve this, health advisors experienced in APS will recruit and enroll 1000 HIV-infected participants at 3 needle-syringe program clinics in the Mathare slum area of Nairobi. Together with peer educators from SAPTA, study health advisors will inform partners of the exposure to HIV and HCV and arrange for testing at the clinic or another venue. This strategy has not been used among PWID for HIV or HCV, however, it has recently been shown to be acceptable and feasible in the general population in Kenya. Our second and third aims leverage the ability of APS to identify new HIV and HCV cases and use phylogenetics to characterize modes of transmission and risk factors for ongoing HIV and HCV transmission in this key population. Blood spots will be collected on filter paper from 1000 HIV-infected and 1000 HCV-infected PWID in the network and HIV and HCV will be sequenced at the University of KwaZulu-Natal research laboratory. This type of molecular epidemiology approach has not been used in this population for HIV or HCV and holds great promise when combined with collection of sociodemographic and behavioral data for defining population level factors contributing significantly to onward transmission of these two viruses. We anticipate that our results will shape the success of future prevention efforts in Kenya and sub-Saharan Africa by enabling public health programs to target high-risk clusters, and this will in turn pave the way for more in-depth research on effective screening and service delivery PWID.