"Hay fever. Peanut allergies. Eczema. Either you have a frustrating allergy, or you know someone who does. Billions of people worldwide-an estimated 30 to 40 percent of the global population-have some form of allergy; millions have one severe enough to actively endanger their health. Even more concerningly, over the last decade, the number of people diagnosed with allergy has been steadily increasing. Medical anthropologist Theresa MacPhail, herself...