Hay fever. Peanut allergies. Eczema. Billions of people worldwide have some form of allergy; millions have one severe enough to seriously endanger their health. And over the past decade, the number of people diagnosed with allergy has been steadily increasing, an ever-growing medical burden on individuals, families, and our health care system. Medical anthropologist Theresa MacPhail, herself an allergy sufferer whose father died of a bee sting, set out to understand why.