I think a lot of this is "provable" with anecdotal evidence, but I think there is something about allowing people to get sick or exposed to germs...we live in a world where people are afraid of germs and sickness and want to drop an antibiotic at the first sign of illness, bacterial or not. I currently live in a country where antibiotics can be gotten at pharmacies without a prescription. When people start to cough or otherwise display symptoms of being sick the first recourse is an antibiotic; doctors even will prescribe antibiotics when the symptoms point towards virus, just to make induce a placebo effect and make sure their patient feels cared for.
Anecdotal evidence...I grew up in the country and was exposed to a lot of "pathogens". My mother was a school nurse (likely bringing some of her work home with her) and just being outside in the woods, going to the bathroom, climbing trees, swimming, stick fighting, making forts, etc...all without washing hands. I never had allergies as a child and the only time I get sick in Beirut is when I am exposed to an extreme amount of pollution or the strange pollen from the pine trees here that plagues nearly everyone about once a year. I am even tolerant of something called the spring worm (caterpillar) that has feces and body hairs that are extremely irritating.
Our bodies are stronger than we give them credit for and we need to stop being a nation/world of hypochondriacs. Its nice that there seems to be some serious study on this issue. From personal experience I seem to see that the people who are the sickest are those that were sheltered the most growing up.