This dissertation consists of three essays in the field of health economics. The first essay provides the first causal evidence that bargaining power in a relationship shapes pregnancy outcomes and health disparities in the US. A key driver of bargaining power is the availability of potential non incarcerated male partners in the local dating market, which I define at the race by cohort by county level. Because these sex ratios are endogenous, I use a novel instrument that leverages the randomness in sex at birth and the persistence of local demographics to isolate exogenous variation in the relative availability of men. Greater female bargaining power causes better outcomes: fewer out-of-wedlock births, less chlamydia and hypertension among mothers, and fewer infants with APGAR score below the normal level. The marriage market makes a significant contribution to racial disparities in pregnancy health. Specifically, Black women face relatively poor prospects when looking for a partner compared to White women: while there are 102 White men per 100 White women, only 89 Black men are available per 100 Black women. According to my estimates, Black women’s disadvantage accounts for 5-10% of the large racial gap in maternal and neonatal health. The racial difference in male availability is mostly policy-driven, as incarceration accounts for 45% of the gap. A counterfactual policy equalizing county-level incarceration rates for non-violent offenses between Black and White people would prevent 200-700 adverse pregnancy outcomes per year among Black mothers through the bargaining power channel alone. The second essay investigates how reopening hotels and ski facilities in Poland impacted tourism spending, mobility, and COVID-19 outcomes. We used administrative data from a government program that subsidizes travel to show that the policy increased the consumption of tourism services in ski resorts. By leveraging geolocation data from Facebook, we showed that ski resorts experienced a significant influx of tourists, increasing the number of local users by up to 50%. Furthermore, we confirmed an increase in the probability of meetings between pairs of users from distanced locations and users from tourist and non-tourist areas. As the policy impacted travel and gatherings, we then analyzed its effect on the diffusion of COVID-19. We found that counties with ski facilities experienced more infections after the reopening. Moreover, counties strongly connected to the ski resorts during the reopening had more subsequent cases than weakly connected counties. The third essay studies the diffusion of influenza-like illnesses (ILI) through social and economic networks. Using almost two decades of weekly, county-level infection and mortality data from Poland, it studies within and across-counties ILI transmission. Firstly, it evaluates the causal effect of school closures on viral transmission. The results show that closing schools for two weeks decreases the number of within county cases by 30-40%. The decline in infections extends to elderly and pre-school children. In addition, flu-related hospitalizations drop by 7.5%, and mortality related to respiratory diseases among the elderly drops by 3%. Secondly, the paper demonstrates the significant contribution of economic links to diffusion across counties. The disease follows the paths of workers commuting between home and workplace. Together with the structure of the labor mobility networks, these results highlight the central role of regional capitals in sustaining and spreading the virus.