e depend on water to live. But we also pollute our water by using it to take our waste away. Dirty, smelly liquids trickle down shanty alleyways; city sewers empty into rivers; and big factories dump their waste into the sea. We all do it.

Chemicals and oil kill wildlife. We’ve all seen the pictures of big pollution disasters from oil tankers. But for humans the big killer is sewage. When it pollutes drinking water, it brings epidemics of diseases like cholera and typhoid, and diarrhoea.

Doctors say that, at any one time, half of the poor people in developing countries in Africa and Asia are sick because of diseases caused by dirty water.

That’s mainly because some 2.4 billion people do not have proper sanitation, whether a flush lavatory attached to a sewer pipe or a properly designed dry lavatory or cesspit. Every day, around a billion people squat at squalid pit latrines in shanty towns. And another billion make do with fields or streams, railway lines or roadsides, buckets or plastic bags.

Many women and girls, shamed by this, wait till the night time before they go at all.

The United Nations has promised to try and halve the proportion of people without safe sanitation by the year 2015. But however you do your toilet, doctors say the most important thing is to wash your hands afterwards – in the cleanest water you can find.