Billions of people are held back by a lack of basic sanitation.

Sanitation, some environmental and health historians say, was the greatest single medical advance in the last 150 years, with efforts to separate sewage from drinking water and promote hygienic practices (hand washing) greatly advancing health and wellbeing in the now-wealthy world. But, as water-quality campaigners and sanitation specialists are saying today, billions of people, mainly in the poor South, are held back by a lack of this basic benefit.

Andrew C. Revkin of the New York Times speaks to journalist Rose George about the book, “The Big Necessity: The Unmentionable World of Human Waste and Why it Matters,” which explores the persistent, and avoidable, sanitation gap. Click here to read more on this story.