Viruses are a major threat to human health, and—given that they spread through social interactions—represent a costly externality. This article addresses three main questions: (i) what are the unintended consequences of economic activity on the spread of infections; (ii) how efficient are measures that limit interpersonal contacts; (iii) how do we allocate our scarce resources to limit the spread of infections? To answer these questions, we use novel high frequency data from France on the incidence of a number of viral diseases across space, for different age groups, over a quarter of a century. We use quasi-experimental variation to evaluate the importance of policies reducing interpersonal contacts such as school closures or the closure of public transportation networks. While these policies significantly reduce disease prevalence, we find that they are not cost-effective. We find that expansions of transportation networks have significant health costs in increasing the spread of viruses, and that propagation rates are pro-cyclically sensitive to economic conditions and increase with inter-regional trade. JEL Codes: I12, I15, I18, H51, C23.