About 600,000 people in Kenya's capital live in the Kibera slums, East Africa's largest informal settlement.
UN Secretary General Ban Ki-Moon has been to the slum on his first visit to Africa, assessing challenges the UN faces in trying to reduce poverty.
Congestion is a major problem. There is not even the space to build toilets.
Some frustrated residents use polythene bags commonly known as "flying toilets" to dispose of human waste.