Allergies are on the rise, experts know. What they don't know is why.
The most popular theory is the so-called hygiene hypothesis. It holds that our culture's addiction to cleanliness, antiseptics and antibiotics prevents our immune systems from developing the ability to ward off real infections. Our bodies then end up overreacting to things they should be ignoring.
In support of this theory are observations that developed countries, which tend to be more squeaky clean, have higher rates of allergies than developing ones, where families are bigger and kids get exposed to more infections.
But not all data fit with the hygiene hypothesis. The theory breaks down in crowded, low-income urban areas in the U.S. Kids there are exposed to more germs. Yet their rates of allergy and asthma remain high.