For hundreds of millions of the world’s poor, home is not a place of comfort but an incubator for disease and despair.

The U.N. Human Settlements Program estimates that more than 1 billion rural dwellers and 600 million urban residents in developing countries live in overcrowded housing with poor water quality, lack of sanitation, and no garbage collection. Roofs and walls of scavenged materials do not protect from rain, wind, pests, or intruders. Dirt floors turn to mud in the rainy season, putting those who cannot afford shoes, chairs, or a bed at greatest risk.