This dissertation studies the impact of public policies on health outcomes related to infectious disease and substance use. Chapter 1: Nursing home residents face both a high risk of influenza and influenza-related mortality. Influenza vaccinations can reduce these risks, but take-up remains suboptimal, in part because people may ignore the spillover benefit of positive externalities of vaccination on disease transmission. One approach that states have taken is to mandate influenza vaccination for nursing home residents and/or healthcare workers. This paper estimates the effect of such state policies on vaccination take-up and influenza-related diagnoses and deaths. I find that resident influenza vaccination requirements increase the probability of vaccination take-up by about 6% and decrease the probability of having an influenza-like illness diagnosis by roughly 20%. Chapter 2: State policies to optimize prescriber use of Prescription Drug Monitoring Programs (PDMPs) have proliferated in recent years. Prominent policies include comprehensive mandates for prescriber use of PDMP, laws allowing delegation of PDMP access to office staff, and interstate PDMP data sharing. This study assesses the effects of three PDMP policies on adverse opioid-related hospital events among patients with prescription opioid use. Comprehensive use mandates were associated with a relative reduction in the probability of opioid-related hospital events by 28% among patients with any opioid and 21% among patients with long-term opioid use. Delegate laws and interstate data sharing were associated with limited change in the outcome. Chapter 3: This chapter investigates the effects of Affordable Care Act facilitated Medicaid expansions on the use of pre-exposure prophylaxis (PrEP) and medications used to treat Human immunodeficiency Virus (HIV). We exploit state-level variations in expansion status to estimate difference-in-difference models. We find a roughly 31% increase in the utilization of PrEP and a 19% increase in the utilization of therapeutic HIV medications within the Medicaid population. We do not find statistically significant evidence that the increased utilization of PrEP following these public insurance expansions had any effect on new HIV diagnoses. We also do not find evidence that the increase in utilization of therapeutic HIV medications was associated with a reduction in HIV deaths.