Public health experts say one of the problems associated with living in a slum—too many people in close proximity—can be a benefit. A single intervention can simultaneously improve many lives in one densely packed community. They call it the “neighborhood effect.” Massive slums have become major features of cities in many low- and middle-income countries. The nearly one billion people who live in slums are a marginalized group facing unique health issues.

“The neighborhood effect in slums is both a problem and an opportunity. It is a problem because it is likely to amplify health hazards,” says Richard Lilford, a professor at the University of Warwick. “However this effect is likely to offer economies of scale and increasing returns on investments to create a healthy environment.

Read more: How to improve the lives of 1 billion people in slums – Futurity