Harvard economist Edward Glaeser is an expert on how cities function as economic engines and centers of innovation. He notes that the advantages of density in spurring creativity and productivity are mirrored by the vulnerability it creates to threats like disease. Cities and their most vulnerable residents have borne the brunt of pandemics since antiquity. As Covid-19 tests the resources and resilience of urban centers and confronts leaders with difficult choices, Glaeser explains the policy options for protecting people and stabilizing the economy.