: Although it has been established that population density can contribute to the outbreak of the COVID-19 virus, there is no evidence to suggest that economic activities, which imply a significant change in mobility, played a causal role in the unfolding of the pandemic. In this paper, we exploit the particular situation of Sardinia (Italy) in 2020 to examine how changes in mobility due to tourism inflows (a proxy of economic activities) influenced the development of the COVID-19 pandemic. Using a difference-in-differences approach, we identify a strong causal relationship between tourism flows and the emergence of COVID-19 cases in Sardinia. We estimate the elasticity of COVID-19 cases in relation to the share of tourists to be 4.1%, which increases to 5.1% when excluding local residents. Our analysis suggests that, in the absence of tools preventing the spread of infection, changes in population density due to economic activities trigger the pandemic spreading in previously unaffected locations. This work contributes to the debate on the complex relationship between COVID-19 and the characteristics of locations by providing helpful evidence for risk-prevention policies.