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Food allergies are on the rise across the world. In the U.S., an estimated two kids in every classroom have a food allergy. Yet doctors still don’t know what causes the immune system to overreact to certain foods. Suspects range from antibiotics to lack of vitamin D to the “hygiene hypothesis,” which suggests we’ve made our environment too sterile. What they do know: Even a trace of an allergen can cause a severe reaction, epinephrine is the only effective treatment for anaphylaxis, and educating the public is one of the best ways to avoid accidental exposure.