Men push goods past a pile of uncollected garbage in the Kibera slum in Nairobi, Kenya: Kibera, which houses almost 1 million people, is the largest slum in Africa, and one of the biggest in the world. It has only two main water pipes, and in most of the slums there is no sanitation, with one hole-in-the ground latrine being shared by up to 50 shacks

Photograph: KAREL PRINSLOO/AP