Allergies are far more common in rich nations than in poor ones. Hygiene hypothesists believe that's because people in poor countries are riddled with parasites. Studies in Africa and South America have tested groups of parasite-infected people and found them to have few or no allergies. These people were then treated with vermifuges. Once they became parasite-free, they developed allergies, Pathologist Marie-Helene Jouvin says. "We don't know precisely how it works, but parasites have developed this very fine mechanism that allows them to survive in a host's body without killing the host and without the host killing them."