Hay fever. Peanut allergies. Eczema. Either you have an 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. Even more concerning, over the last decade the number of people diagnosed with an allergy has been steadily increasing, placing an ever-growing medical burden on individuals, families, communities, and healthcare systems.

Medical anthropologist Theresa MacPhail, herself an allergy sufferer whose father died of a beesting, set out to understand why. In pursuit of answers, MacPhail studied the dangerous experiments of early immunologists as well as the mind-bending recent development of biologics and immunotherapies that are giving the most severely impacted patients hope. She scaled a roof with an air-quality controller who diligently counts pollen by hand for hours every day; met a mother who struggled to use WIC benefits for her daughter with severe food allergies; spoke with doctors at some of the finest allergy clinics in the world; and discussed the intersecting problems of climate change, pollution, and pollen with biologists who study seasonal respiratory allergies.

This is the story of allergies: what they are, why we have them, and what that might mean about the fate of humanity in a rapidly changing world.