OVER 7,000 strong and growing, community farmers’ markets are being heralded as a panacea for what ails our sick nation. The smell of fresh, earthy goodness is the reason environmentalists approve of them, locavores can’t live without them, and the first lady has hitched her vegetable cart crusade to them. As health-giving as those bundles of mouthwatering leafy greens and crates of plump tomatoes are, the greatest social contribution of the farmers’ market may be its role as a delivery vehicle for putting dirt back into the American diet and in the process, reacquainting the human immune system with some “old friends.”

Increasing evidence suggests that the alarming rise in allergic and autoimmune disorders during the past few decades is at least partly attributable to our lack of exposure to microorganisms that once covered our food and us. As nature’s blanket, the potentially pathogenic and benign microorganisms associated with the dirt that once covered every aspect of our preindustrial day guaranteed a time-honored co-evolutionary process that established “normal” background levels and kept our bodies from overreacting to foreign bodies. This research suggests that reintroducing some of the organisms from the mud and water of our natural world would help avoid an overreaction of an otherwise healthy immune response that results in such chronic diseases as Type 1 diabetes, inflammatory bowel disease, multiple sclerosis and a host of allergic disorders.

In a world of hand sanitizer and wet wipes (not to mention double tall skinny soy vanilla lattes), we can scarcely imagine the preindustrial lifestyle that resulted in the daily intake of trillions of helpful organisms. For nearly all of human history, this began with maternal transmission of beneficial microbes during passage through the birth canal — mother to child. However, the alarming increase in the rate of Caesarean section births means a potential loss of microbiota from one generation to the next. And for most of us in the industrialized world, the microbial cleansing continues throughout life. Nature’s dirt floor has been replaced by tile; our once soiled and sooted bodies and clothes are cleaned almost daily; our muddy water is filtered and treated; our rotting and fermenting food has been chilled; and the cowshed has been neatly tucked out of sight. While these improvements in hygiene and sanitation deserve applause, they have inadvertently given rise to a set of truly human-made diseases.