In March this year the Department of Economic and Social Affairs, Population Division, of the United Nations published World Urbanization Prospects: The 2009 Revision – Highlights. There’s a very nice chart in this:
The message is clear: sort rural sanitation and stop open defecation, but the long-term problem is going to be sanitation in low-income high-density urban areas, including slums. It’s bad enough now, but it could well get a whole lot worse before it gets better – and it’ll only get better if politicians in developing countries wake up to the realities of urban poverty in their own back-yards.