Medary.com

Science Daily:

“The natural immune system does not have as much to do as it did 50 years ago because we’ve increased our efforts to protect our children from dirt and germs,” says McMorris.

“Allergies are on the rise because our society has changed the way we live. As a result, people with allergies are having children with others who have allergies, which in turn creates a natural increase in the prevalence of allergies in our society.”

Allergies are a reaction by the body's immune system to foreign substances – pollen, mold, animal dander, dust and dust mites, insect stings and certain foods – that it deems  harmful.

I've heard of this before, and am pretty sympathetic to this argument.  We evolved in a dirty, nasty world.  Maybe Mr. Clean is causing our sniffles?