Our current public health research aims to advance HIV prevention and treatment research by contributing new methodology to study causal mechanisms in networks of people who use drugs at high risk for HIV infection. We also use cutting-edge causal inference and network science methods to address the prevention and treatment of opioid use disorder using routinely-collected data. Recently, we are developing epidemiologic models for the transmission of SARS-CoV-2 in New York City to better account for underlying social network structures that are spatially resolved and evaluate interventions aimed at sheltering at-risk populations from COVID-19 and, ultimately, reducing health disparities.