The scientists also sought to reproduce these immunological profiles in animals by treating mice with microbe-laden dust from both Amish and Hutterite homes. The two dusts had drastically different effects when the mice inhaled them through their noses every few days for over a month. Amish dust prevented symptoms of asthma; Hutterite dust encouraged them.

Broadly speaking, the immune system has two arms: the adaptive immune system, which learns and remembers; and the innate immune system, which operates like a sensory organ, recognizing ancient patterns in the microbial world. When the scientists genetically hobbled the animals’ innate immune systems, the Amish dust lost its protective effect, and the animals began to have trouble breathing. The implication is that stimulation of the innate immune system is critical to preventing asthma.

The study has some shortcomings. It’s small — just 30 children from each community. The scientists didn’t identify the specific microbes that might be important. Nor do they know if those microbes take up residence in the gut microbiome or elsewhere in the body. Martin Blaser, director of the Human Microbiome Program at New York University, also points out that the scientists didn’t control for antibiotic use or C-section rate, both of which may, by disturbing the gut microbiota, alter asthma risk.

But the fact that they could so faithfully reproduce in mice what they saw in people using only dust suggests that they’ve identified an important component of the farm effect. And the simplicity of the mechanism — microbes that stimulate the innate immune system — is heartening. “That is precisely why we’re so excited,” Donata Vercelli, a researcher at the University of Arizona in Tucson and a senior author on the study, told me. “This seems to be a manageable situation,” she said, one that could lead to a plausible intervention, like a preventive medication based on Amish microbes.

The findings also reiterate the theme that genes aren’t destiny. Disease emerges from the dance between genes and environment. The asthma epidemic may stem, at least in part, from the decline of what Graham Rook, an immunologist at University College London, years ago called our “old friends” — the organisms our immune systems expect to be present in the environment. The newly sneezing upper classes in the 19th century may have been the first to find themselves without these old friends. Now most of the developed world has lost them. The task at hand is to figure out how to get them back. One answer may come from the Amish cowshed.