This project, conducted in collaboration with preeminent HIV scholars at Johns Hopkins, uses multiple modeling frameworks to examine interaction between HIV prevention interventions delivered in a combination package among high-risk populations in India, particularly persons who inject drugs (PWID), to determine the optimal combination of prevention and treatment interventions. Using data from two ongoing large, community randomized trials of interventions to prevent HIV in multiple sites in India, investigators will develop compartmental and network models of HIV transmission (incorporating detailed human contact network structure or simplified representations of contact) to disentangle the multiple dynamic effects of combination interventions. These modeling frameworks will be used to assess the current suite of interventions currently recommended for PWID - including HIV counseling & testing, antiretroviral therapy, opioid substitution, needle exchange, treatment of sexually transmitted infections, treatment of tuberculosis, education and condoms - but can be extended to incorporate other intervetions (e.g., PrEP) and other hard-to-reach populations in India.