What Makes Viruses Like COVID-19 Such a Risk for Human Beings? The Answer Goes Back Thousands of Years

The Faroe Islands are little volcanic spits of land poking out of the far north Atlantic Ocean. On the outer edges of Europe, they are isolated and cold; in 1846 they were one of the healthiest places on earth. But in that year, a carpenter, a native of the island, returned from Copenhagen with a bad cough. He had the measles. The virus had been absent from the Faroe Islands for more than 60 years, and, in the days before a measles vaccine, few of the island’s residents had immunity to the disease. Over the course of the next five months, 6,100 of the island’s 7,900 inhabitants fell ill. Over a hundred died.

Island populations are natural laboratories for epidemiology — the study of the population dynamics of infectious disease. And the measles epidemic of 1846 is of unusual interest because it marks the beginning of modern epidemiology. The Danish government, the distant sovereign over these tiny islands, dispatched Peter Panum, an observant young doctor with mutton-chop sideburns, to the scene. His report proved beyond a doubt that the disease was spread by direct contact. It is a seminal document of epidemiology.

Notably, the measles epidemic on the Faroes burned itself out in the space of five months. When the virus ran out of new hosts, the chain of infection was extinguished. Herein lies one of the most important lessons of the epidemic. The measles virus has a “critical community size” of 250,000, meaning that without a dense human population above that threshold, the virus will naturally drive itself to extinction, with nobody nearby left to get it and pass it on. The survival of the measles virus, as with any parasite, is intricately connected to the population dynamics of its host.

The history of a respiratory virus like measles can help to put the current coronavirus pandemic in perspective. Humans are distinctive animals in the number and nature of our pathogens — and especially in the number of respiratory viruses that infect us. COVID-19 is a new contender to join the permanent roster, but we are watching a pattern in human history repeat itself before our eyes.

What we often fail to recognize is that Homo sapiens serves as host to an unusual number of respiratory viruses. The sniffling, sneezing, coughing afflictions that stalk us from birth to old age are a distinct feature of life as a human being. In all, humans are afflicted by dozens of respiratory viruses that have evolved to specialize in the exploitation of us. That’s strange. Consider the disease pool of our closest primate relative, the chimpanzee. Because of our genetic relatedness, humans and chimpanzees have similar immune systems. Yet only about two dozen viruses have ever been identified in chimps, and many of these are in fact human viruses that have transiently infected chimpanzee populations (sometimes with horrific effect) when exposed to human observers. Chimps are the natural host of only a few viruses, and these are on the whole rather benign. Most of the parasites that are adapted to chimpanzees are worms or protozoa. The reasons for this pattern have to do with their population sizes and lifestyles. Chimps live in small groups and move frequently, which makes it impossible for specialized respiratory viruses that cause acute disease to adapt to them. In short, our chimpanzee cousins — who live in the jungle, eat raw monkey for breakfast, never bathe and make a habit of chewing on their own feces — suffer only a fraction of the viral diversity that we do.

The distinctive human disease pool — and its array of respiratory viruses in particular — is a product of our distinctive history as a species. We are the sneezing ape. A few million years ago, our ancestors would have been afflicted by a typical group of primate parasites, much like the ones chimpanzees still harbor today. But then the hominins mastered fire and underwent a dramatic series of physiological changes that made us different from other apes.

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So long as early humans were foragers — dependent on hunting and gathering for subsistence — population sizes were limited and human groups were small. The hunter-gatherer germ pool was probably closer to the chimpanzee germ pool than to the modern human collection of infectious diseases. The first truly great turn came with the Neolithic Revolution, starting around 12,000 years ago. The invention of agriculture in various regions of the globe transformed human societies and their germ pools. More food meant more people, and populations exploded. The domestication of animals brought us into closer contact with zoonotic pathogens, some of which evolved the ability to infect humans. But the great respiratory viruses that are such a feature of human history were not an immediate consequence of the Neolithic Revolution. For millennia, human population numbers were too low, and human settlements were too small, to sustain the transmission of most highly virulent respiratory diseases.

But in the Iron Age, the period of great empire-building across Eurasia some two to three thousand years ago, settlements in China, South Asia, the Near East, and the Mediterranean were scaled up. Ancient Rome became the first city with a million inhabitants. Maybe the trickiest evolutionary challenge that all parasites have to confront is how to transmit between hosts. Like measles, COVID-19 is caused by a virus that seems to transmit primarily via droplets expelled when victims cough or sneeze. Respiratory transmission generally requires close contact and large population sizes. With humans living closer together, it became easier than ever for pathogens to solve that challenge, jumping from host to host, from lung to lung, on droplets floating in the air. If we equate the history of “civilization” with the story of these societies capable of building large cities, then the history of civilization is synonymous with a period in which a clever primate started to collect a strange number of lung-jumping viruses that would have otherwise passed quickly into oblivion. Even against the backdrop of the brief 300,000 years or so that constitute the entire history of humanity, most of the familiar respiratory viruses established themselves in human populations within the last 1% of that span.

Most of this history goes unrecorded in traditional documents, or at best is remembered in vague references to unidentifiable plagues and pestilences. But increasingly, the hidden history of human disease is illuminated by genetic evidence. The family tree of the measles virus, for example, is telling, revealing that today’s measles is descended from a rodent disease that jumped to cattle and subsequently to humans. There are sophisticated mathematical methods for estimating the timescale of these evolutionary relationships. In the case of measles, one new study (to which I was a contributor) estimates that measles diverged from Rinderpest, a now-extinct cattle virus that is its closest relative, in the later first millennium BC. This estimate centers almost exactly on the moment in human history when the largest cities first passed the “critical community size” for the establishment of measles virus.

What this alignment suggests — disturbingly enough — is that one of the most dangerous and distinctly human respiratory viruses emerged instantaneously with the rise of civilization itself. The broader implication may be that humans are under constant assault, and that human social development has stoked the evolution of our many pathogens, enabling them to avoid the hasty extinction that would have otherwise awaited them. Our very success in taking over the earth and commandeering its resources has made us attractive hosts.

The deep history of human disease, then, can help us understand that the current coronavirus pandemic is part of our unique trajectory as a species. At least seven species of coronaviruses — some of them mild and prevalent, some of them rare and virulent — can infect humans, using our lungs as the staging ground for their schemes of genetic replication. The new coronavirus may establish itself permanently in the global network of human lungs, or like SARS coronavirus, it might be stopped in its tracks by our massive global public-health interventions. At this stage, it seems as if the former is more likely, and COVID-19 will be the latest addition to our all-too-human roster of human pathogens. The virus is novel, but the pattern is old. As we encroach on nature, and expand toward eight billion, the pattern will continue to repeat itself. It is the plague of our success as a species.

Historians’ perspectives on how the past informs the present

Kyle Harper is a historian at the University of Oklahoma and author of The Fate of Rome: Climate, Disease, and the End of an Empire. He is writing a new history of infectious disease.