One of the most urgent problem in the world to day is shelter, in other words, homes for people to live in. In many cities, especially in the Third World, tens of thousands of people live in shacks which they have built themselves out of pieces of wood and iron, old boxes and oil drums. Many cities are surrounded by unplanned settlements, sometimes called shanty towns, where the poorest people live. In these areas sewerage and safe water supplies are inadequate and, as a result, the inhabitants are likely to catch illnesses and diseases.