Christophe Harerimana, a school teacher for more than 20 years in Rwanda, became worried in recent years. Many of his students were coming to class ill and were having trouble concentrating.

“The children were having abdominal pain, diarrhoea and nausea. This made it difficult for them to follow the lessons,” says Harerimana, who teaches science and Kinyarwanda, the local language of his village, in the Musanze District.

Harerimana later discovered that many of the children had soil-transmitted helminth infections, which impair long-term physical and cognitive development. In 2007, nearly 95% of school-aged children in the Musanze district were infected—one of the highest rates in the country.