The developed world is in the midst of an “allergy epidemic." Yet while half of Americans are at a higher risk of developing some sort of allergic disease, not even a tenth of children in an Indiana Amish community are sensitized to common allergens.

The rate of allergic disease and asthma has increased by almost three-fold over the 20th century — one in five American children now have a respiratory allergy and almost 10% suffer from asthma. Conversely, “the Indiana Amish [are] among the least allergic populations ever described in the developed world,” science writer Moises Velasquez-Manoff, wrote in a recent opinion article in The New York Times.

While genetics is undoubtedly behind this discrepancy, it doesn’t explain the whole mystery. According to Mark Holbreich, an allergist who works in Indianapolis, a clear majority of the Amish children he tested have had extensive exposure to a rural setting. He suspects that contact with farming must somehow make their immune systems more resilient, a phenomenon known as the “farm effect.”

The idea is that a rural setting introduces children to microbes, mold, plant material and raw milk, which challenge their immune system early on in life, making it alert and ready for whatever invaders they run into later on.

Manoff noted that a boost in allergic disease is now happening in quickly developing countries too — regions that rarely had them before. A case in point is China where a 2009 study found significantly lower rates of allergic sensitization and asthma rates among schoolchildren in rural areas outside of Beijing compared to those who lived in the city.

Another sign that our modernized surroundings diminish the resilience of our immune systems is the fact that immigrants who move from a developing part of the world are altogether less allergic but become more allergic the longer they live in a developed country.

Studies have discounted differences in exposure to urban pollution as well as levels of physical activity as factors that explain this immunological deviation. In fact, strengthening of the immune system is likely happening before birth when pregnant mothers are exposed to these not-so-sterile rural environments.

One study found that the likelihood of a child developing eczema lessens as the amount of microbes found in the mother’s mattress increases. Children have also been found to develop allergic disease much less frequently when they are born to mothers who worked with livestock when pregnant.

The immunological cell that might steer this trend is a type of white blood cell, regulatory T-cells, which keeps the immune system in line and not over-reactive. One Munich University study discovered that babies of farmers have more regulatory T-cells in their umbilical cord blood than those born to nonfarmers.

“[T]hat suppressive ability increases with the number of different types of animals the mother tended while pregnant,” Manoff wrote. “The more cows, pigs, and chickens a mother encounters, essentially, the more easily her offspring may tolerate dust mites and tree pollens.”

The good news is that our immune systems can be responsive to the farm effect until as late as young adulthood, according to a Danish study.  But Manoff warns city folk that occasional, brief visits to your uncle’s farm might actually have the opposite effect and exacerbate allergic responses. Apparently, the farm effect requires the young to settle into country living for a while.

Altogether, farmers' well-trained immune systems probably resemble what everybody's immune system was like as it evolved in a microbial-rich hunter-gatherer setting. What this means, scientists say, is that our immune systems have become wimpier thanks to the cleaner, modern luxuries that have pampered us since the Industrial Revolution.