SHEOHAR DISTRICT, India — He wore thick black eyeliner to ward off the evil eye, but Vivek, a tiny 1-year-old living in a village of mud huts and diminutive people, had nonetheless fallen victim to India’s great scourge of malnutrition.

His parents seemed to be doing all the right things. His mother still breast-fed him. His family had six goats, access to fresh buffalo milk and a hut filled with hundreds of pounds of wheat and potatoes. The economy of the state where he lives has for years grown faster than almost any other. His mother said she fed him as much as he would eat and took him four times to doctors, who diagnosed malnutrition. Just before Vivek was born in this green landscape of small plots and grazing water buffalo near the Nepali border, the family even got electricity.

So why was Vivek malnourished?

It is a question being asked about children across India, where a long economic boom has done little to reduce the vast number of children who are malnourished and stunted, leaving them with mental and physical deficits that will haunt them their entire lives. Now, an emerging body of scientific studies suggest that Vivek and many of the 162 million other children under the age of 5 in the world who are malnourished are suffering less a lack of food than poor sanitation.

Like almost everyone else in their village, Vivek and his family have no toilet, and the district where they live has the highest concentration of people who defecate outdoors. As a result, children are exposed to a bacterial brew that often sickens them, leaving them unable to attain a healthy body weight no matter how much food they eat.