Whenever Bill Gates visits a slum, an unasked question goes around his mind.

“Do you mention to each other that it doesn’t smell good, and I wouldn’t like to live here, or is that just inappropriate?” he told me in a phone interview. “We take for granted that we have a toilet right inside our household.”

In fact, it’s a luxury. Only 27 per cent of the world’s population has a home toilet that sends waste to sewers, then on to a treatment plant, estimates the World Health Organization. Three people in 10 have neither toilets nor latrines. They pay a daily price in disease and lost dignity. “Sanitation-related diseases, including diarrhoea, cholera and typhoid, kill nearly 500,000 children under age five, each year,” says the Gates Foundation. But now, thanks largely to Gates, companies are about to start selling what he terms the “reinvented toilet”. He sounds as proud of it as of the Microsoft Windows operating system that made him rich. The reinvented toilet could transform countless lives, and might even take off in wealthy countries.