Pandemic: Tracking Contagions, from Cholera to Coronaviruses and Beyond

by Sonia Shah

As wetlands were paved over and forests were felled, different species came into novel, prolonged contact with each other, allowing animal microbes to spill over into human bodies. And these developments were proceeding at an unprecedented scale and speed around the world. The road from animal microbe to human pathogen was turning into a highway.

But avian biodiversity, like the biodiversity of other species, has plummeted, in the United States as elsewhere. Urban sprawl, industrial agriculture, and climate change, among other disruptions caused by human activity, steadily destroy bird habitats, reducing the number of species among us. But habitat destruction doesn’t affect all species equally. Some species—the so-called specialist species—get hit especially hard. They’re the ones, like monarch butterflies, salamanders, and woodpeckers and rails, that rely on exacting conditions and can’t easily survive when those conditions change. When trees are felled and nesting grounds paved over, they’re the ones that tend to disappear first. That means there’s more food and territory around for the “generalist” species like robins and crows—the opportunistic, sharp-elbowed types that can live anywhere and eat anything. Their numbers skyrocket in the vacuum.

As avian diversity declined in the United States, specialist species like woodpeckers and rails disappeared, while generalist species like American robins and crows boomed. (Populations of American robins have grown by 50 to 100 percent over the past twenty-five years.)48 This reordering of the composition of the local bird population steadily increased the chances that the virus would reach a high enough concentration to spill over into humans.

The loss of species diversity in northeastern forests of the United States similarly allowed tickborne pathogens to spill over into humans. In the original, intact northeastern forests, a diversity of woodland animals such as chipmunks, weasels, and opossums abounded. These creatures imposed a limit on the local tick population, for a single opossum, through grooming, destroyed nearly six thousand ticks a week. But as the suburbs grew in the Northeast, the forest was fragmented into little wooded plots crisscrossed by roads and highways. Specialist species like opossums, chipmunks, and weasels vanished. Meanwhile, generalist species like deer and white-footed mice took over.

But as comfortable as these medical tourists may be, once under the knife their internal tissues will be exposed to New Delhi’s unique microbial environment, and they bring any microbes they gain during their procedures back home. People who undergo surgeries are especially vulnerable to infectious pathogens. Surgeons’ knives breach the protective layer of skin that separates the interior of the body from the exterior environment, allowing the army of microbes that live on the surface of the skin, in the air above the bed, and on the surgical instruments and other objects that pass over the open wound to slip in. Even the most elaborate sterilization practices often fail to stop them. Those that make their way into the body are likely to flourish, since the surgery itself, not to mention the conditions that often precede it, depress patients’ immune systems.

The act of excretion itself didn’t require privacy or provoke shame back then as it does now. Sixteenth- and seventeenth-century monarchs such as England’s Elizabeth I and France’s Louis XIV openly relieved themselves while holding court.

Far from reviling human feces, medieval Europeans even began to think of it as medicinal. According to a history of sanitation by the journalist Rose George, the sixteenth-century German monk Martin Luther ate a spoonful of his own feces every day. Eighteenth-century French courtiers took a different route, ingesting their “poudrette,” dried and powdered human feces, by sniffing it up their noses.

But far from being a harmless source of fertilizer, dog feces is both an environmental contaminant (and is classified as such by the Environmental Protection Agency) and a source of pathogens that can infect people. Like human excreta, dog poo teems with pathogenic microbes, such as strains of E. coli, roundworms, and other parasites. One of the most common parasitic infections in Americans is the result of their exposure to dog feces. The dog roundworm Toxocara canis is common in dogs and, because of the ubiquity of dog feces, widespread in the environment. It can contaminate soil and water for years.