The prevalence of allergic respiratory diseases, asthma and allergic rhinoconjunctivitis, has increased since the advent of industrialization. The inverse relationship between the number of infections early in life and atopy has been interpreted as the “hygiene hypothesis.” That is, many infections early in life promote the development of T helper type 1 cytokines, while fewer infections early in life favor the development of T helper type 2 (Th2) cytokines and atopy. An alternate interpretation of the same data, that atopy is protective against infections early in life, is rarely considered. With epidemiologic, historical, and immunologic data, I suggest that human evolution has favored individuals with an atopic predisposition. Th2 immune responses promote parity, and ensure successful pregnancy and term birth; provide the infant protection against infections and the inflammation induced by common pathogens in the first years of life until the immune system matures; and protect young adults exposed to viral respiratory pathogens. These traits are of particular value with the advent of industrialization, especially so in the era prior to the development of antibiotics. This theory contradicts the assumption that there is no biological or evolutionary advantage for allergic disease to exist in humans and has significant implications for our current and future treatments of allergic diseases.