We know from COVID-19 that proximity between people is a major risk factor in the spread of outbreak disease - and yellow fever is no exception. As increasing numbers of people in Africa and South America move to cities, the risk of mass-outbreaks of yellow fever increases. Slums, favelas and other informal dwellings are particular risk hotspots. This episode hears from Dr Erin Staples from the United States Centers for Disease Control and Prevention, and Dr Matthew Steele of the Bill and Melinda Gates Foundation.