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What If Humans Are the Real Pandemic?

In the past 100 years, well over 335 new infectious diseases have appeared. Looking at the relationship infectious diseases have with hosts, the environment, and intermediaries, I think we need to consider an unpleasant possibility.

Living under imposed isolation, my mind often drifts to the coronavirus and our narrative surrounding it. Our simplistic way of understanding the crisis betrays our belief in an uncomplicated world not found in the natural world. We, the golden-hearted protagonists, struggle against the virus, the obvious and unquestioned villain. We assure ourselves we will triumph.

We emerged from the tropics, spread across the earth, exhausting resources, displacing the species we regard as lesser. Our invasive possession of the world upset the delicate balances that have existed since time immemorial.

If the disorder brought by a living organism capable of spreading defines an infectious disease then, unpleasant though it is, we must contemplate whether Homo sapiens became a pandemic infectious disease.

The Missing Third Wheel

Parasitic infection likely conjures up horrific images of flaccid, endless worms threatening to exit your posterior. Maybe it calls to mind the invasive growth that happens when a bot fly entrusts the future of its species to your epidermis. The distinct sense of an “other,” the offensive presence of a free-loading organism, that is where we begin.

Our relatively pristine guts are a recent development in terms of evolution. The intestinal tracts of our ancestors' likely housed a diverse range of microbes and organisms, not unlike our primate relatives today. Their relatively stable health starkly contrasts how we might fare with the same infectious diseases living in and on our person.

Did our ancestors lead miserable and sick existences? Mostly, probably not, at least not as modern humans would. Long-established ecosystems — specifically those untouched by the explosive human population growth if such a place exists anymore — often contain a highly specific, balanced dynamic that lurks beneath the surface.

Take the relationship between trypanosomes, antelope, and the tsetse fly, for example. Trypanosomes, a protozoan parasite, transported into the human body makes its way to your brain causing devastating neurologic disease that eventually kills if not treated. The disease is called sleeping sickness. Yes, apparent sleepiness is one symptom — in case you were on the edge of your seat.

Trypanosome in rat blood | Alan R Walker

Now, imagine yourself in the savannah of Africa where the tsetse fly and the antelope make their home. In search of food, you have the option of hunting antelope, but the parasite means you risk catching a deadly disease. Would you risk it or search for a lower-stakes lunch elsewhere? It may have taken a few generations to work it out, but humans eventually searched for less costly meals.

Neither the antelope nor the fly suffered any ill effects from the infection, but in exchange for the hospitality, the parasite made areas inhospitable to humans. Indigenous people, living in the area where the parasitic trio call home, adapted to the threat.

Instead of farming like the surrounding areas, people in the sleeping sickness region maintained a hunting culture. The regions remain remarkably undeveloped and without the agricultural influence in the 21st century. For much of that, the area can thank the parasitic trypanosome.

The relationship between the fly, antelope, and parasite is a dynamic common in highly diverse environments like tropical forests. The entire animal world exists in a balance preventing any group from multiplying out of control. Environments like temperate jungles maintained a stable balance of predator and prey. It's likely this balance was the norm prior to human disruption.

The Breakup

The life cycles and how it fits into the parasite-host relationship may include a third party, an intermediate host, like with malaria and the mosquito. The third party is optional and some organisms like worms can directly enter the host.

What the relationship between the freeloading microbe and the host looks like determines if the environment could end the dynamic. That may end an outbreak, or it could end the relationship between the species forever.

No mosquito would likely mean the end of the troubled relationship between humans and the insect. parasite to continue infecting people so it can reproduce inside the body of a human.

Parasites, especially the single-celled variety, can make their living in the humid soil on the rainforest floor. Here parasites may break from their hosts, latching on to their next home when the opportunity comes along. It’s unlikely the host species can avoid infection so long as it remains in the environment.

Like the trypanosomes and antelope, we probably boasted an assortment of parasites without apparent disease. Under stress or after a serious injury, the barriers that contained parasites could fail. The ballooning parasitic population could then cause disease and even death.

These age-old relationships developed over thousands and thousands of years where one party changing often guaranteed the other adapted to the change. If one party leaves the environment, especially suddenly, the relationship might end between the two.

Language seemingly willed us into our new existence as hunter-gatherers, defining us. Still, the rest of the world remained much the same. Probably more than a few parasites lost their hosts, thanks to our newfound hunting skills and communication. With these, we moved into a strange terrain.

Logging in lowland rainforest in Sabah, Borneo

Organisms that spread readily between humans could have traveled with us to the plains. Those requiring the nurturing forest floors would have to stay. It’s possible they might still be waiting for us there now.

Food for Thought

Although our ability to cooperate majorly upset the initial order of things, we didn’t exit the framework. We were an evolutionary rags-to-riches tale. Unlike other predators, we could strategize in hunting. The advantage fueled our brains’ expansion.

The same moving parts interacted in roughly the same way despite our meteoric rise. Though language and cooperation neutralized the threat from other predators, the advantage was not enough to overcome all limitations, namely infectious diseases.

The sheer number of human infectious diseases that exist in Africa probably isn’t a coincidence. The crucible that forged modern man should also be the location with formidable means of controlling our extreme advantage.

We recognize this dynamic in other species readily, where predator and prey populations ebb and flow in step with one another. Overpopulated species that live too closely together become the welcoming substrate for a rapidly spreading disease that could reduce the population.

Nothing suggests we were an exception to the natural order. Distancing to control the spread of coronavirus makes perfect sense when we understand our origins. The change removes a weakness that nature might use to curtail our continued growth.

We would not begin that growth until we gained two more assets: Fire and clothing.

Domesticating fire allowed us to escape the tropical regions that tethered us before. Cooking jettisoned the expensive task of digestion. Now, we could meet the demands of our increasingly large brains. Homo erectus, the human model that came after Homo habilis, possessed a brain that was 50% larger than H. habilis. Only cooking allowed for explosive growth.

Human Evolution 101 | Nat Geo

The Emperor’s New Clothes

Developed clothing only improved our ability to tolerate harsher climates. Now, we could wear the skins of animals adapted to those new locations. Instead of living as a tropical species bound to a tropical space, we were now a tropical species capable of going anywhere, and we did so in the extreme. Humans could rightly be called the ultimate invasive species.

What followed our acquisition of fire and clothing differed from the first shift where we moved from just another primate to the apex predator. Removing the climate constraints transformed our relationship with the world. Humans traveled to unknown places with fewer parasites and diseases that previously curtailed our exponential growth.

In an evolutionary blink, the megafauna fell beneath a human population tsunami that no longer checked into the system that balanced it. The last remaining giants probably owe their continued existence to parasites like the trypanosomes that encouraged humans to go elsewhere.

For a while, it seemed humanity would simply expand until the earth could hold no more. Of course, leaving the bondage of our old environment did not mean we left the world.

We densely populated cities, displacing many highly specialized relationships like that of the trypanosome, antelope, and the fly. Our invasive growth knocked loose the cogs of finely tuned systems that existed in the places humans overtook.

We purchased the world when it was not for sale and did so on credit without thought of repayment. Perhaps, we thought, “who could ever hold us to account?” The earth itself, it would seem. The debt collector has come for payment, with interest, in full.


Public health biologist studying at Johns Hopkins | Science writer & artist | Views reflect me alone | Subscribe @

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