In the old days—that is, a few weeks ago—I would often share my mornings with my neighbor Wesley. We’d greet each other with a hug, then head across the street to the garden plot we share, in a leafy neighborhood of Washington, D.C. The garden used to be a landfill, left behind when a row of houses was bulldozed a few decades ago. Over many months, we have improved the soil with kitchen compost and leaf mulch inhabited by a mob of microbes, insects, and worms. During this year’s unseasonably warm spring, we planted some seedlings and harvested kale and mustard, then prepared a fresh salad for lunch. Wes and I have grown close through these routines, even though I’m in my mid-fifties and he’s in his late twenties and we share little in common other than a small tuft of urban territory.

Most of the metaphors we have for talking about our biological world do not match this model of coöperation. Darwinian thought—or the popular cartoon version of it—teaches us the concept of never-ending competition between the “fit” and the “unfit.” Abrahamic religions tell us that human beings were given the earth and its creatures to rule. American mythology encourages entrepreneurial individualism. But Wesley and I do not compete for space in our small raised garden bed; instead, we share microbes from the air and the soil, expelling them on our breaths and wiping them onto our hands and, later, ingesting them. With our actions, we form a community, in both a social and microbial sense.

Microbial webs have bridged the spaces between human beings and other species for all of our history. Long before anyone knew what a single-cell organism was, cultural practices maximized the exchange of microbes: as people farmed, foraged, tended livestock, fermented their food, dipped their hands in common bowls, and greeted one other with a touch, they engaged in rituals that bound them together with their neighbors and other organisms. This was probably not accidental. A wealth of evidence shows that, when we share microbes with other people and organisms, we become healthier, better adapted to our environments, and more in synch as a social unit.⁠

The interconnectedness of our biological lives, which has become even clearer in recent decades, is pushing us to reconsider our understanding of the natural world. It turns out that the familiar Linnaean taxonomy, with each species on its own distinct branch of the tree, is too unsubtle: lichens, for example, are made up of a fungus and an alga so tightly bound that the two species create a new organism that is difficult to classify. Biologists have begun questioning the idea that each tree is an “individual”—it might be more accurately understood as a node in a network of underworld exchanges between fungi, roots, bacteria, lichen, insects, and other plants. The network is so intricate that it’s difficult to say where one organism ends and the other begins. Our picture of the human body is shifting, too. It seems less like a self-contained vessel, defined by one’s genetic code and ruled by a brain, than like a microbial ecosystem that sweeps along in atmospheric currents, harvesting gases, bacteria, phages, fungal spores, and airborne toxins in its nets.

In the midst of the coronavirus outbreak, this idea of a body as an assembly of species—a community—seems newly relevant and unsettling. How are we supposed to protect ourselves, if we are so porous? Are pandemics inevitable, when living things are bound so tightly together in a dense, planetary sphere?

The history of civilization has hinged on the building and demolition of boundaries between species. Early agriculture disregarded most of the natural world in order to cultivate only the most productive plants and animals; this allowed populations to grow and cities to flourish. But crops and livestock, once they were concentrated in one place and cultivated in monocultures, became vulnerable to disease. As cities and farm operations grew, people and animals crowded closer together. The result was a new epidemiological order, in which zoonotic diseases—ones that could jump from animal to human—thrived.

At first, these diseases remained confined to the places where they originated. Then globalization arrived. John McNeill, an environmental historian at Georgetown University, speculates that the first wave of the cholera outbreak of 1832-33 was the first true pandemic; it reached every inhabited continent by hitching rides on caravans and ships. More infections followed, often affecting the crops on which people depended for food. In the early nineteenth century, potato plants in South America suffered from a blight; the culprit, a mold called Phytophthora infestans, sailed to Ireland in 1845, where it led to a million deaths. In the eighteen-sixties, a tiny aphid-like bug called phylloxera migrated from the United States to Europe, nearly pulping the French wine industry; in the nineteen-sixties, Panama disease eradicated the world’s favorite commercial banana, the Gros Michel. In 1970, the fungus Bipolaris maydis decimated the American Corn Belt before spreading worldwide; another fungal infection, wheat rust, has caused countless famines worldwide.

And yet the upsides of industrial agriculture were hard to resist. In the nineteen-fifties, the Green Revolution churned out so many cereal crops that the United States began giving food away; when its techniques were exported to the rest of the world, they defused the “population bomb.” In the sixties, the American-led Livestock Revolution vertically integrated the production of animal products, creating a parallel increase in the consumption of meat. By the seventies, big poultry companies were churning out so many chickens that they had to invent new products—chicken nuggets, chicken salad, chicken-based pet food. Large corporations bought up local producers of poultry, pork, and beef; feedlots grew to the size of fairgrounds; hen houses dwarfed neighborhood strip malls. Farms went from being small operations with an average of seventy chickens to factories housing thirty thousand birds. In the eighties, with the Blue Revolution, the industrial farming of fish expanded, too. From 1980 to 2018, the global production of animals for consumption grew about one and a half times faster than the world population.

Barns packed with animals are good places to breed pathogens. Monocultures, in which all animals are genetically similar, offer few speed bumps to transmission. “You got fifty thousand chickens in a barn,” Rob Wallace, the author of “Big Farms Make Big Flu,” told me. “They are all genetically the same and you are growing them for a turn-around time of six weeks. That is all food for flu.” Normally, pathogens evolve to be harmful but not deadly: they want to co-opt hosts without killing them, so that they can continue their spread. But, in the fast-paced world of an industrial hen house, where birds come and go quickly, pathogens select for the most virulent strains, no matter how deadly. Within the uniform predictability of modern agriculture, the unpredictable emerges.

Zoonotic diseases can seem like earthquakes; they appear to be random acts of nature. In fact, they are more like hurricanes—they can occur more frequently, and become more powerful, if human beings alter the environment in the wrong ways. The Centers for Disease Control and Prevention estimates that three-quarters of the “new or emerging” diseases that infect human beings have originated in wild or domesticated animals.⁠ In addition to the familiar pathogens—Ebola, Zika, avian flu, swine flu—researchers have counted around two hundred other infectious diseases that have broken out more than twelve thousand times over the past three decades.⁠ It’s no small feat to cross the species barrier; these numbers speak to the scale of our agricultural system.