Asthma rates have been surging around the globe over the past three decades, and for a long time researchers thought they had a good idea of what might be fueling the increase: the world we live in is just a little too clean.

According to this notion--known as the hygiene hypothesis--exposure in early childhood to infectious agents programs the immune system to mount differing highly effective defenses against disease-causing viruses, bacteria and parasites. Better sanitary conditions deprive the immune system of this training, so that for reasons that are still unclear, the body pounces on harmless particles--such as dust and ragweed--as if they were deadly threats.

The resulting allergic reaction leads to the classic signs of asthma: chronic inflammation or swelling of the airways and ac...