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The Invisible Killers

Humankind has made so much progress in bending nature to our will that we sometimes forget our own place in it. The history of pandemics shows that the proverbial fourth horseman of the apocalypse – pestilence – can never be vanquished, only contained.

MILAN – In 1969, the US surgeon general, William H. Stewart, told Congress that it was time “to close the books on infectious diseases” and “declare the war against pestilence won.” Antibiotics, vaccines, and widespread advances in sanitation were making the world healthier than ever. Within a few years, the medical schools at Harvard and Yale actually closed their infectious-disease departments. By then, polio, typhoid, cholera, and even measles had essentially been eradicated, at least in the West.

But triumphalism was not only premature; it was dangerously foolhardy. The HIV/AIDS epidemic broke out in the United States just a decade later, and never has been vanquished. Then, following a short lull in the 1990s, came SARS, MERS, Ebola, Zika, and avian and swine flu, to name just a few of the outbreaks so far this century. Though most of these new diseases have primarily afflicted the poorest parts of the world, they should have made clear that the war on microbes was far from over.

Nonetheless, a sense of invulnerability has prevailed in the West. It was assumed that even if epidemics had not been consigned to history, they posed a risk only to geographically and economically distant societies. The novel coronavirus that emerged in Wuhan, China in December has shattered this illusion, showing once again that novel pathogens are equal-opportunity killers.

After initially deceiving ourselves that COVID-19 would remain just another Asian health crisis, the entire world is now grappling with a runaway pandemic. Suddenly, public-health authorities everywhere are trying to flatten the contagion curve with quarantines, travel bans, and unprecedented society-wide lockdowns, while governments and central banks try desperately to flatten the recession curve with unprecedented stimulus packages.

Disease and Denial

One lesson is already clear: Even in the richest, most advanced economies, humans are still humans, which means they are vulnerable to new microbial threats, particularly zoonotic infections (diseases that spread from non-human animals) resulting from natural evolution and facilitated by human activities. As two recent histories of pandemics show, it is always only a matter of time before a virus, bacterium, or parasitic organism makes the leap from some non-human species to our own.

Ebola, for example, came from chimpanzees, just as bubonic plague emerged from rats and COVID-19 (most likely) from bats. And, in addition to worrying about new microbes, we also must worry about older ones. Owing to antigenic mutations, malaria and tuberculosis, once almost defeated, have reemerged in drug-resistant forms.

In Epidemics and Society, the Yale University historian Frank M. Snowden shows why the West’s complacency was never justified. Far from being the exclusive preserve of “backward” societies, deadly disease outbreaks are, if anything, a negative byproduct of human progress. By altering ecosystems and erasing natural frontiers, humans have continuously exposed themselves to germs, viruses, and bacteria that evolve to exploit their vulnerabilities. The push of economic development has brought more opportunities for humans and animals to intermingle, and globe-spanning trade has established new routes for the propagation of disease.

Epidemics and Society

In recorded history, the battle between humans and microbes has essentially been a fight between reason and superstition. For centuries, human societies felt powerless in the face of pandemics, so they resorted to religious rituals to placate some supposedly irate god. When science eventually triumphed over religion, one illusion was replaced by another. We convinced ourselves that we were the gods, capable of conquering nature and the microbial world.

By examining this long and painful learning process, we can better understand why the world was so unprepared for the current crisis. Snowden confidently takes the reader on a wide-ranging journey, tracing the history of the major pandemics that have afflicted the world – from bubonic plague, smallpox, and cholera to tuberculosis, malaria, polio, HIV, and Ebola. Snowden’s goal is to show how humanity learned to tame infectious diseases through the creation of public-health systems and the progress and dissemination of medical knowledge.

This has been a constant struggle, in part because each infectious disease has been unique. For some, the key trait was their contagiousness; for others, it was their deadliness. Some were bacterial, others viral or parasitic. Some were transmitted by air, others through contaminated water or vectors like fleas, mosquitoes, and lice.

What they all shared was the ability to inflict severe suffering on humans and cause far-reaching disruption of entire societies. Infectious diseases have a unique capacity to fuel anxiety, fear, mass hysteria, and outbursts of religiosity (especially in the past). They pose a direct challenge to social cohesion and solidarity, and thus to a society’s ability to manage collective crises.

Peak Pandemic

Of all the landmark infectious outbreaks that Snowden analyzes, the bubonic plague remains the most emblematic for its epidemiology, persistence, and effects on society. Even more to the point, it permanently influenced how health authorities deal with infectious diseases.

Remembered for its virulence, lethality, and horrifying clinical manifestations, the plague killed 50% of those infected within days of the onset of symptoms. And, unlike polio, measles, mumps, and other diseases that tend to strike children and the elderly, the plague targeted adults in the prime of life, leaving behind many widows and orphans – and thus magnifying the economic, demographic, and social dislocations.

Moreover, the plague is the only highly infectious disease that has continuously ravaged the world throughout the last 1,500 years, accompanying humanity from the age of religious superstition into the age of scientific hubris. It usually emanated from Africa or Asia and then spread to Europe and America with the help of globetrotting merchants. Recurring epidemic waves lasted for decades or even centuries.

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The Black Death, for example, wiped out up to one-third of the European population between 1334 and 1372, and then returned intermittently until as recently as 1879. Meanwhile, the other infectious diseases that emerged during this period – from syphilis in the 1490s to cholera in 1830 – ran in parallel with it.

For centuries, Europeans believed that the plague was sent by an angry deity as punishment for disobedience and sin. To placate divine wrath, communities often sought to scapegoat and cast out the supposed sinners, be they prostitutes, Jews, religious dissenters, foreigners, lepers, beggars, or accused witches.

During the plague years of the fourteenth and fifteenth centuries, towns across Europe closed themselves off to outsiders and hunted down anyone within their walls who was deemed undesirable. Those who were apprehended were stoned, lynched, and burned at the stake.

A less cruel response focused on propitiating the angry god through penitence. A typical example was the outdoor procession to a holy shrine amid rogations and confessions, like those organized by the Flagellants who traveled across Europe before being persecuted by the Inquisition.

In other cases, afflicted communities resorted to the cult of saints who were supposed to intercede with God on behalf of suffering humankind. In recognition of Mary’s intervention to end the plague in 1631, for example, the city of Venice built the monumental church of Saint Mary of Health at the entrance to the Grand Canal.

Religious fanaticism clearly was not enough to defeat the plague, given its continuous recurrence. But other anti-plague measures, many of them draconian, represented some of the first institutionalized forms of public-health policy.

During the Black Death, Italian cities pioneered plague regulations that cloaked health authorities with emergency powers and facilitated coordination between the army and the bureaucratic apparatus. Within cities, the ill were isolated in pest-houses or locked in their homes with guards at the doorstep. The military was delegated to isolate the population with sanitary cordons to prevent the inflow of disease-carrying people and goods. Venice was the first city to quarantine ships and their crews.

 In the event, these early anti-plague policies marked a key moment in the emergence of the modern state. Deadly outbreaks justified top-down measures to control the economy and the population through forcible detention, surveillance, and the suspension of liberties. The same containment and social-distancing measures would remain the first line of defense against almost any infectious disease, from cholera and yellow fever to HIV and Ebola. And though these policies were not always appropriate or effective, they conferred an image of decisive leadership on rulers.

Scientific Hubris

Despite the prevailing superstitions, our ancestors understood that the plague was transmitted from person to person, and that isolation was necessary to contain the contagion. Still, they did not know what caused the disease. For centuries, doctors, influenced by the so-called miasma theory, believed that “bad air” emanating from rotting organic matter was the source of sickness.

Only in the eighteenth and nineteenth centuries did physicians begin to understand what was really happening beneath the surface. The invention of the microscope led to the germ theory of disease, which identified microorganisms – not miasma – as the source of infection. In the case of the plague, the culprit was the bacterium Yersinia pestis, which was carried by fleas living on the black rats that were a constant presence in the crowded cities and merchant ships of the time.

The emergence and widespread acceptance of germ theory represented a turning point in the fight against infectious disease, ushering in a medical revolution and the creation of entire new fields such as microbiology, immunology, parasitology, and tropical medicine. By the mid-twentieth century, the most prevalent and aggressive infections were in retreat, thanks to the discovery of vaccines and antibiotics, higher living standards, and improved hygiene.

Vaccination alone reduced the incidence of smallpox, diphtheria, tetanus, rubella, measles, mumps, and polio to such a radical degree that these diseases have been largely forgotten. The chemical DDT was poised to eradicate malaria and other insect-borne pathogens until it was found to be carcinogenic. And cholera was more or less knocked out by sand filtration and the chlorination of water.

In The Pandemic Century, a vivid account of the scientific community’s fight against viruses over the last century, the science journalist Mark Honigsbaum shows how these achievements produced a sense of mastery over the microbial world. After centuries of suffering at the hands of capricious gods, humanity suddenly began to develop seemingly divine powers of its own.

The Pandemic Century

But with that realization came hubris. In 1948, US Secretary of State George Marshall confidently declared that humanity was about to eradicate infectious diseases from the Earth. To his generation, microbes were seen as static or slowly evolving, geographically constrained, and thus eminently manageable. Old diseases were being wiped out, and few stopped to consider that new ones might arise.

As is obvious to us now, this idea of microbial fixity – that there can be only so many diseases – was misplaced. Since 1940, scientists have identified 335 new infectious diseases, two-thirds of which originate in wildlife, particularly bats. Familiar examples include Lassa fever, Marburg virus, Lyme disease, Rift Valley fever, West Nile virus, SARS, MERS, Nipah virus, and Ebola, but there are many, many more.

Whenever dangerous pathogens are defeated, it is only a matter of time before others take their place. New diseases are the inevitable condition of living in a dynamic world. Human beings are part of an immensely complex ecological system. Bacterial and viral infections can lie dormant in tissue and cells – or under now-melting permafrost – for decades before being reactivated by a sudden shock to the system, or through co-infection with another microbe.

In 2013, for example, Simon Anthony of Columbia University and his team discovered that the number of novel viruses in all mammalian species could be around 320,000, with bats being the most common carriers because they live in large communities, travel long distances, and are widespread throughout the world. The line that divides infectious from chronic diseases is also increasingly blurred. Papillomavirus, for instance, is the primary cause of a number of cancers affecting both men and women.

Moreover, according to a 2016 report by the US National Academy of Medicine, “the underlying rate of emergence of infectious diseases appears to be increasing.” Many more diseases are emerging from animal reservoirs and ecological niches that used to be far removed from human populations. Demographic growth, climate change, crowded cities, persistent poverty, and global trading routes continue to disrupt fragile ecological equilibriums and expose humanity to the threat of new killer pathogens.

Proceed With Caution

In Epidemics and Society, Snowden points to a 1998 report from the US Department of Defense warning that, “Historians in the next millennium may find that the twentieth century’s greatest fallacy was the belief that infectious diseases were nearing elimination. The resultant complacency has actually increased the threat.” Just two decades later, that prediction has been borne out, with rich and poor countries alike brought to their knees by a coronavirus pandemic.

That said, it is not the scientific community that bears all of the blame for our miscalculations. After the exuberance of the 1960s and 1970s proved untenable, virologists, epidemiologists, international organizations, and non-governmental organizations have understood that pandemics are still an acute threat. In 2015, the philanthropist Bill Gates sounded the alarm about the world’s lack of preparedness for a flu pandemic. But policymakers and business leaders were too busy reaping the fruits of unfettered globalization to heed the warnings.

To be sure, no other recent epidemic has threatened global health and the economy on the scale that COVID-19 has. The World Health Organization did warn in 2009 that the swine flu (H1N1) met the criteria for a pandemic virus. But the risk of global disruption did not materialize. Similarly, in 2003, SARS was expected to become a new influenza pandemic, but that turned out to be a false alarm. While a single sneeze can set a pandemic in motion, the intra-species contingencies are complex, making large-scale pandemics low-probability events.

But low-probability does not mean no probability. The COVID-19 pandemic has exposed our vulnerability and lack of preparedness, underscoring the need for a more cautious approach in the future. As with all infectious outbreaks, its suddenness has sown confusion and chaos. The psychological, economic, and social damage it has inflicted will lead to permanent changes in our economies, politics, and individual lives.

New pestilences will emerge without warning in the future. But one hopes we will prepare for them without adopting an apocalyptic mindset or indulging in the scapegoating of the Middle Ages. Living under perpetual pandemic alert would strain our livelihoods and limit our freedoms. And besides, there is a third way. Hubris should defer to humility. Our scientific aspiration should be to understand the microbial world, not to conquer it.

Governments, for their part, should heed the insights that science offers. By adopting more sustainable economic paradigms, strengthening public-health systems, restoring faith in experts, and developing resilience against negative shocks, we can minimize the likelihood of another pandemic-driven global catastrophe.

No matter how deep our understanding of the microbial world, Snowden and Honigsbaum remind us, nature will always furnish viruses, bacteria, and parasites with endowments we didn’t anticipate. After the COVID-19 pandemic has passed, theirs and other books on infectious diseases are likely to remain open for quite some time.

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