Viruses are a major threat to human health, and – given that they spread through social interactions – represent a costly externality. This papers addresses three main issues: 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 their spread? To answer these questions, we use novel high frequency data on the prevalence of a number of viral diseases across space, for different age groups,  over a period of a quarter of a century.  We use quasi-experimental variation to evaluate the importance of policies reducing inter-personal contacts such as school closures or  the closure of public transportation networks. We evaluate the health costs induced by the expansion of transportation infrastructures. Finally, we find that propagation rates of viruses are pro-cyclical and are increasing with trade.