Here, the farm effect dovetails with the burgeoning science on the prenatal origins of disease generally. What happens to your mother during the nine months before your birth may affect your vulnerability to many diseases decades later, from heart disease and obesity to schizophrenia.

Allergies and asthma seem to follow the rule as well.

Susan Prescott, a doctor and researcher at the University of Western Australia in Perth, has noted differences in the placentas of children who later develop allergies. A critical subset of white blood cells — called regulatory T-cells — seems relatively scarce at birth. Rather than enabling aggression, these cells help the immune system restrain itself when facing substances that are not true threats. A healthy population of these and other “suppressor” cells is important, scientists now suspect, in preventing allergies and asthma. So it seems significant that European farming children are born with a comparative surfeit of these cells. Bianca Schaub, a doctor and researcher at Munich University, has found that farming newborns have more regulatory T-cells in cord blood than babies of nonfarmers. In test tubes, these cells more effectively quash allergic-type reactions. And that suppressive ability increases with the number of different types of animals the mother tended while pregnant. The more cows, pigs and chickens a mother encounters, essentially, the more easily her offspring may tolerate dust mites and tree pollens.

Animal studies demonstrate how this might work. Some years back, scientists at Philipps University of Marburg in Germany sprayed pregnant mice with microbes originally isolated from Bavarian cowsheds. The exposure induced favorable changes in gene expression at the placenta. The pups born to these mice were protected against asthma.

This research suggests that farming mothers might benefit from a naturally occurring immunotherapy, one that preprograms the developing fetus against allergic disease. Yet how to apply that therapy deliberately remains unclear. Is “microbial pressure” what matters — a stiff microbial wind in our sails? Or do certain cowshed microbes actually colonize farmers, and favorably calibrate their immune function?

There’s evidence to support both explanations, which aren’t mutually exclusive anyway.

Before you rush to the nearest farm, however, a word of caution. Some studies indicate that if you grow up in an urban environment, occasional visits to the farm may exacerbate allergic propensities. If you haven’t matured with abundant microbial stimulation, the thinking goes, encountering it intermittently may push you into overdrive, prompting the misery you seek to avoid.

And yet, a prospective study from Denmark published this month suggests that it’s never too late. Young adults who began farming (with livestock) were less likely to develop new allergic sensitivities than rural peers who chose other professions. Existing allergies didn’t disappear. Rather, the farming environment seemed to prevent new sensitizations.

Which brings us to farm milk. In Europe, the consumption of unpasteurized milk has repeatedly correlated with protection against allergic disease. In America, 80 percent of the Amish studied by Dr. Holbreich consume raw milk. In a study published earlier this year, Dr. Schaub’s group showed that European children who consumed farm milk had more of those regulatory T-cells, irrespective of whether they lived on farms. The higher the quantity of those cells, the less likely these children were to be given diagnoses of asthma. Here, finally, is something concrete to take off the farm.