Worms infect more than one-third of the world's population, with the heaviest infections in school-age children and the poor. Studies show that children carrying high worm burdens have stunted growth. They also perform poorly in cognitive tests and educational achievement. Fortunately, many of these consequences of worm infection can be reversed through the use of cost-effective drug treatments. An increasing number of countries are now implementing national deworming programmes, in many cases delivered through schools.