Cities are on the frontline of the COVID-19 pandemic. Population density, while essential for productive, healthy and innovative cities, is a catalyst for the virus’ rapid transmission. When combined with poverty, marginalisation and informality, density can put over 1 billion people globally living in city slums and informal settlements at heightened risk. Those urban hotspots are characterised by overcrowding, shared facilities and housing, lack of access to essential services and dependence on informal economies. They may exist in low and middle-income countries as well as in fragile states.