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Allergies were pretty much completely unknown until the beginning of the twentieth century, and allergic diseases have skyrocketed in the last twenty years, and well over half of all Americans are allergic to an airborne substance. What's going on here?

At first glance, that big spike in allergies might seem a bit unlikely — particularly to people like me, who are lucky enough to not have to deal with allergies. What could cause such a massive uptick in the incidence of allergies? Is this a natural byproduct of the modern world we live in, or this allergy scare all in our heads? The answer, it seems, might actually be somewhere in the middle.