Soil-transmitted helminths infect 2 billion people. http://www.who.int/mediacentre/factsheets/fs366/en/

More than a few will benefit, because in many areas with high infection rate, the majority of children really do have worms. Regarding Ascaris infection specifically: "In areas of poor sanitation virtually 100% of the population harbor the parasite."[Markell & Voge's Medical Parasitology p.241]