Ehrlich said that back then, perhaps as far back as 30,000 years before the birth of Christ, our immune systems needed to be powerful to fight off these germs. The people with the strongest immune response survived while others died. "So," he said, "being an allergic person may have been an advantage." (1, page 6)
Yet today we have many defenses against such invaders, such as shoes, clothing, clean drinking water, processed food, vegetables that are treated with pesticides, air conditioned buildings, etc. We receive vaccinations and use hand sanitizers. People today are barely exposed to germs, so the allergic response isn't needed.
For most of us, our immune systems have adapted to the change. Yet for some of us, our immune systems continue to work overtime. Lacking harmful germs to occupy our immune systems, they become bored and develop a sensitization to things that are supposed to be safe, such as dust mites, pollen, molds, and cockroach urine.
So this is the basis, at least one theory anyway, of why about 10 percent of the world's population develop allergies. I would speculate the same holds true to asthma as well.
- Ehrlich, Paul M., Elizabeth Shimer Bowers, "Living with Allergies," 2009, page 6