The recent Covid-19 pandemic highlighted that understanding the channels of disease transmission is crucially important for public health policies. However, measuring transmissions occurring through casual contact in the public space is highly challenging as researchers generally do not observe when infected individuals intersect casually with noninfected individuals. We overcome this methodological challenge in the context of the Covid-19 pandemic by combining card payment data, indicating exactly where and when individuals visited stores, with test data indicating when they were infected. We document that exposure to an infected individual in a store is associated with a significantly higher infection rate in the following week. Our estimates imply that transmissions between retail shoppers made a substantial contribution to the Covid-19 pandemic.