Were workers more likely to be infected by COVID-19 in their workplace, or outside it? While both economic models of the pandemic and public health policy recommendations often presume that the workplace is less safe, this paper seeks an answer both in micro data and economic theory. The available data from schools, hospitals, nursing homes, food processing plants, hair stylists, and airlines show employers adopting mitigation protocols in the spring of 2020. Coincident with the adoption, infection rates in workplaces typically dropped from well above household rates to well below. When this occurs, the sign of the disease externality from participating in large organizations changes from negative to positive, even while individuals continue to have an incentive to avoid large organizations due to the prevention costs they impose on members. Rational cooperative prevention sometimes results in infectious-disease patterns that are opposite of predictions from classical epidemiology.