We live at the crossroads of three global megatrends, three barreling and intertwined juggernauts of modernity. The first is the massive migration of humanity to the world’s cities. I grew up in a small town, walking deer trails beneath the shade of maples and oaks. Now I live with my kids in a city where the path beneath our feet is ever more likely to be paved. My story is our story. By 2050, two-thirds of all humans on Earth will live in cities.

The second is the loss of biodiversity. Species are disappearing, both from the places where we live and from the earth as a whole. If our hairy ancestors were to visit our cities and suburbs, they would wonder how the escalators work, but they would also question where the plants and animals have gone. What have we done with all the birds? Some, like the Carolina parakeet, are just gone. Others live on, but at a distance—geographically removed from our daily lives, far away from the majority of people.

And then there’s the third trend—the one that, at first glance, seems not to belong with the others. The prevalence of allergies and chronic inflammatory diseases among urban populations in developed countries has skyrocketed in recent years. Incidences of asthma, Crohn’s disease, multiple sclerosis, and even depression (which can have an immune component) are on the rise.

The parallels in geography and timing between urbanization, the loss of biodiversity, and the rise in immune-system problems raise an intriguing—and troubling—question. Could our distance from nature and our chronic immunological discontent be related? Some now say . . . yes.

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