Allergies affect over a quarter of the population in Europe. Some allergies manifest with seasonal symptoms of rhinitis and headache, others are immune reactions on exposure to certain foods, plants or animals. Before the concept of allergy was introduced just a century ago, it was largely unknown or restricted to the wealthy classes of society. Investigating the causes for this epidemic, Patrizia Marani, herself an allergy sufferer, talked with the experts and discovered that we have become involuntary subjects of a global experiment, the results of which are only now coming to light.