At the turn of the nineteenth century, Aaron Burr, who would later become the third Vice-President of the United States, secured a charter and millions of dollars in financing to establish the Manhattan Company, which promised to tap clean drinking water for New York City. Instead, Burr and his associates cut costs by drawing water from a putrid pond just south of what is now Canal Street and redirected the remaining money to found a bank (now called JPMorgan Chase). The city’s contaminated water supply contributed to two major outbreaks of cholera, a disease that causes vomiting, diarrhea, and dehydration so extreme that patients can lose ten per cent of their body weight in less than a day.

New Yorkers received tainted water for decades, despite persistent protests over the foul taste. The city only bowed when a group of beer brewers began complaining that unpleasant water was undermining their product. An aqueduct was built that began to draw clean water from the Croton River, in 1842; eventually, cholera all but disappeared from New York. As Sonia Shah demonstrates in her new book, “Pandemic: Tracking Contagions, from Cholera to Ebola and Beyond,” the vicissitudes of political will—along with other environmental factors—can tip the balance between pathogens and humans. If the wrong choices are made, microbes that have lurked on and around us for years can suddenly cause devastating outbreaks. The right choices can maintain an equilibrium in which fewer people get sick.

The pathogens responsible for recent outbreaks of cholera, Ebola, Lyme disease, and Zika are not new. For thousands of years, the bacteria that cause cholera, for instance, have lived in close association in coastal waters with crustaceans called copepods and helped to recycle material from these animals’ hard exteriors. “Were it not for them, copepods’ mountain of exoskeletons would starve the ocean of carbon and nitrogen,” Shah writes. As people moved into copepod territory, however—first in the Sundarbans in South Asia and then elsewhere—the bacteria adapted to their new potential hosts; they evolved the ability to secrete a toxin that facilitates their spread from one person to another by causing diarrhea. Following the 2010 earthquake in Haiti, United Nations troops, some of them infected with the pathogen, arrived from Nepal, bringing the disease with them. As cholera flared in their camp, workers dumped contaminated waste into the river nearby. It was a “bacteriological bomb,” Renaud Piarroux, a French infectious-disease specialist who investigated the outbreak at the behest of the French and Haitian governments, told me. Thousands of people became sick from contaminated water; and the bacteria spread rapidly throughout the country. It was a natural disaster coupled with a manmade one that fuelled the pandemic.

In the case of Lyme disease, physicians spotted a pattern of arthritic symptoms in adults and children in Lyme, Connecticut, as early as 1975; a few years later, they identified the bacterium responsible for the disease. The number of new cases has risen dramatically in recent years, however, in part due to ecological reasons. Suburbanization has brought people into closer proximity with white-footed mice and deer, which carry ticks, and enabled these animals to proliferate; at the same time, the natural predators of deer, like wolves and cougars, have declined in number. Meanwhile, a promising vaccine, which came onto the market in 1998, was soon withdrawn, in part because of low demand, coupled with skeptics’ claims that it caused adverse reactions. (The Centers for Disease Control and Prevention later found that the vaccine did not cause unusual side effects.)

As for Ebola, the virus has been present in Central Africa since at least the mid-nineteen-seventies. It is known to proliferate in the bodies of wild bats, which come into contact with humans on the edge of the forest or around mango trees in yards. But early outbreaks did not affect large numbers of people; and for years, it seemed that major ones were unlikely. (Shah cites a microbiology paper from 2011 that referred to Ebola, in its title, as “a relatively minor public health threat for Africa.”) In 2014, however, the disease exploded in West Africa, killing thousands in Guinea, Sierra Leone, and Liberia. Why this happened still isn’t clear. At the time, Ebola probably was not new to that area. Research from the nineteen-eighties suggests that in one region of central Liberia, up to thirteen per cent of the population carried antibodies to Ebola in their blood and thus may have already been exposed to the virus. Once the outbreak began, however, the population densities of major cities helped to sustain its growth. So, too, did the limited capacity of hospitals in the area; the lack of basic protective gear, like gloves, among some health-care workers; and the slow response of the international community. “The most critical factor was time,” Rob Lever, a specialist registrar in infectious diseases at the Hospital for Tropical Diseases, in London, told me, noting that the World Health Organization took months to declare a public-health emergency.

If every disease is the result, in some way, of political and scientific choices, then why don’t we make better choices? Of course, the factors that drive outbreaks—poor infrastructure, changes in land use, more frequent contact among wild and domestic animals and people—can be addressed. But doing so requires political will and sustained attention. In Haiti, where cholera continues to spread, there is an ongoing need for vaccines, as well as swift diagnosis, treatment, and reporting of disease. The water system also requires large-scale repairs; when it rains, broken pipes allow sewage to leak into the system. Yet dwindling funds have forced programs to scale back. The world’s attention has largely moved elsewhere, Piarroux says. Meanwhile, in the U.S, a highly pathogenic type of avian flu was detected, in late 2014, in poultry, which were probably infected by wild birds. While no human deaths were reported in the U.S, similar strains in Asia have spread directly from birds to people; experts worry that such viruses could acquire the ability to pass from person to person. Yet so far the public has largely yawned: “How many times does the world have to threatened with a deadly pandemic that moves from one species to another before people get the point?” Michael Specter asked on this site last year. Recently, the international community and the U.S. have taken some steps toward faster detection and response to emerging disease, chastened in part by Ebola. But it remains to be seen whether these efforts will be sustained. “The history of U.S. funding is so episodic and memory is so short, it’s hard not to be pessimistic,” Josh Michaud, the associate director for global health policy at the Kaiser Family Foundation, told me.

Paradoxically, there is one major area in which scientists and the public have embraced the role that microbes play in promoting human health—within the microbiome. The bacteria that live in our guts, on our skin, in our lungs, and elsewhere in our bodies contribute to an internal ecosystem that influences our susceptibility to disease. Disturbances to gut bacteria, like those caused by antibiotic use, can allow for the proliferation of drug-resistant Clostridium difficile (much as deer proliferate in upstate New York in the absence of abundant natural predators). Differing signatures of gut bacteria have also been associated with conditions such as obesity, inflammatory bowel disease, and multiple sclerosis (though to date, most studies show correlation, not causation). In 2013, the author Michael Pollan noted that scientists now “ask whether the time has come to embark on a project of ‘restoration ecology’—not in the rain forest or on the prairie but right here at home, in the human gut.” Since 2007, the National Institutes of Health has supported a multimillion-dollar human-microbiome project, which explores how communities of microbes affect health. And in the past few years, research has validated the use of fecal transplants to treat drug-resistant C. difficile infections, further supporting the approach of driving out bad microbes by repopulating their environment with a mix of competitors.

The divergence is striking: at a moment when global warming and other environmental factors affecting health seem more urgent and harder to address than ever, we seem to have doubled down on a kind of privatized ecology. Enthusiasts, including researchers, perpetually oversell the microbiome. And in 2015, Americans purchased over ten billion dollars of probiotics, many of them with unproven benefits. At some point, perhaps our obsessive focus on the microbiome will be matched by attention to the macrobiome and environment around us. But that may only happen when specific business interests are threatened and demand change. That, after all, is the lesson of cholera and Aaron Burr.