Comparing the Effects of STI Prevention The goal of this project will be to extend the population attributable fraction (PAF) work to assess the potential impact of STI screening on HIV incidence. Specifically, this project merges two existing CAMP-supported projects: the PAF model and the STI guidelines model. We will assess the downstream effect of implementation of the STI guidelines on HIV incidence among MSM due to increased risk of HIV transmission and acquisition in the presence of prevalent STI. This work will include the effects of gonorrhea and chlamydia at a minimum. If we are able to overcome the challenges of incorporating syphilis into the model then we will also assess the effects of testing and screening for syphilis.