Should your children pick their noses? Don’t laugh. Scientifically, it’s an interesting question. Your body needs to know what immune challenges lurk in the immediate environment.

“I tell people, when they drop food on the floor, please pick it up and eat it,” said Dr. Meg Lemon, a dermatologist in Denver who treats people with allergies and autoimmune disorders. “Get rid of the anti-bacterial soap. Immunize! If a new vaccine comes out, run and get it. I immunized the living hell out of my children. And it’s OK if they eat dirt.”

She’s referring to the fact that our immune system can become disrupted if it doesn’t have regular interactions with the natural world. “Our immune system needs a job,” Lemon said. “We evolved over millions of years to have our immune systems under constant assault. Now they don’t have anything to do.”

Leading physicians and immunologists are reconsidering the antiseptic ways in which we interact with our environment.

Why? Let us turn to 19th-century London.

The British Journal of Homeopathy, Volume 29, published in 1872, included a prescient observation: “Hay fever is said to be an aristocratic disease, and there can be no doubt that, if it is not almost wholly confined to the upper classes of society, it is rarely, if ever, met with but among the educated.”

Hay fever is a catchall term for seasonal allergies to pollen and other airborne irritants.

More than a century later, in November 1989, another influential paper was published on the subject. The author looked at the prevalence of hay fever among 17,414 children born in March 1958. Of 16 variables the scientist explored, he described as “most striking” an association between the likelihood that a child would get hay fever allergy and the number of his or her siblings.

It was an inverse relationship, meaning the more siblings the child had, the less likely it was that he or she would get the allergy. Not just that, but the children least likely to get allergies were ones who had older siblings.

The paper hypothesized that “allergic diseases were prevented by infection in early childhood, transmitted by unhygienic contact with older siblings, or acquired prenatally from a mother infected by contact with her older children.

“Over the past century declining family size, improvements in household amenities, and higher standards of personal cleanliness have reduced the opportunity for cross infection in young families,” the paper said. “This may have resulted in more widespread clinical expression of atopic disease, emerging in wealthier people, as seems to have occurred for hay fever.”

This is the birth of the hygiene hypothesis. The ideas behind it have since evolved, but it provides insight into a challenge that humans face in our relationship with the modern world.

Our ancestors evolved over millions of years to survive in their environments. For most of human existence, that environment was characterized by extreme challenges: scarcity of food or food that could carry disease; unsanitary conditions, withering weather, and so on. It was a dangerous environment.

At the center of our defenses was our immune system, our most elegant defense. The system is the product of centuries of evolution. Late in the process, humans learned to take steps to bolster our defenses.

We took care to avoid certain foods that experience showed could be dangerous or deadly. In some cultures, people came to avoid pork, which we now know is susceptible to trichinosis; in others, people banned meats, with we later learned may carry toxic loads of E. coli and other bacteria.

Our ideas evolved, but for the most part, the immune system did not. This is not to say that it didn’t change. When we encounter various threats, our defenses learn and then are much more able to deal with that threat in the future.

We survived over tens of thousands of years. Eventually, we washed our hands, swept our floors, cooked our food, purified our water, and developed waste treatment plants; we isolated and killed bacteria and other germs.

The immune system’s enemies list was attenuated, largely for the good. Now, though, we have created a mismatch between the immune system — one of the longest surviving and most refined balancing acts in the world — and our environment.

Thanks to all the powerful learning we’ve done as a species, we have minimized the regular interaction not just with germs but even with friendly bacteria and parasites that helped to teach and hone the immune system — that “trained” it.