In 2007, Indi Trehan was approached by a Malawian woman whose two children were severely malnourished. Trehan, a paediatrician from Washington University in St Louis, had seen many such cases. Malawi has one of the highest rates of child mortality in the world, and half of these deaths are due to malnourishment. Trehan had come to the country to help.

But these children surprised him. They were identical twins. One had a type of malnutrition called marasmus, and looked emaciated and skeletal. The other had kwashiorkor, a very different condition where fluids leak from blood vessels and don’t get reabsorbed. These children have puffy swollen limbs, distended stomachs and irritable moods. Their skin starts to break down and