“Generals always prepare to fight the last war” is a common military adage, invoked to remind leaders to plan for the future. Currently, we are immersed in what could be described as World War III. It’s not a battle in which humans fight and kill each other but are instead uniquely united in trying to defeat an infinitesimally tiny infectious agent. We already know that the economic toll will probably be trillions of dollars. We can only hope that the death toll does not approach the 1918 Spanish flu’s estimated 50 million.

Two aspects of the struggle against this new viral agent merit further attention: how and where it originated, and how best to fend off — if not prevent — the next pandemic. And there will most assuredly be a next one.

Human history and — and undoubtedly its prehistory — is pockmarked with recurrent plagues and pestilence: the Plague of Athens (429–426BC, probably due to typhus, a bacterium), the European Black Death (1331–1353; caused by the plague bacteria), HIV/AIDS (1920 to the present, a virus), and Ebola (2013–2016, also a virus). All told, these relatively well-documented epidemics have killed about a half billion people.

To be sure, humankind has now made remarkable progress in the struggle against infectious diseases: Edward Jenner advanced vaccine theory in the late 18th century; Louis Pasteur developed an understanding of germs in the mid-19th century; and Alexander Fleming discovered the effectiveness of penicillin in curing bacterial maladies in the early 20th century.

But their victories were never final. Within his lifetime, Fleming witnessed the rise of drug-resistant bacteria, while the development of successful vaccines against some devastating viruses remains unrealized.

About 20 years ago, I teamed up with a colleague to write a book about the deadly rise of drug-resistant bacteria. I still remember why one publisher refused our proposal: “you are not physicians!” My reply: “humanity’s never-ending battle with deadly pathogens is as much about evolution as it is about medicine.” Indeed, pathogens like bacteria and viruses and protozoans (such as malaria) are not only the products of evolution but continue to evolve, often very rapidly. In his 1994 bestseller The Hot Zone, Richard Preston recounted the origins of terrifying hemorrhagic fevers like Ebola and Marburg. Quoting biologist Russell Mittermeier’s “human meat market theory, ” he explained how a predatory pathogen seeking an abundant food source is incentivized, in evolutionary terms, to develop the means to prey on Homosapiens (population: 7.6 billion and increasing) rather than, say, pangolins (population less than 10,000 and certainly declining).

Which brings us to our present plight. The 14th century “Black Death,” or bubonic plague as it was also called, was caused by the bacterium Yersinia pestis, which is carried by infected fleas, themselves carried by rats. Plague is a zoonotic disease, an infectious malady which spreads from animals to humans. Anthrax, malaria and COVID-19 are other examples. A fascinating aspect of zoonotic diseases is that their animal hosts can remain healthy, posing little threat to our species when left alone. But when we invade or destroy their habitat, or when we cram them into filthy cages in fetid urban wildlife markets, they do pose a threat: the diseases they carry may then invade, sicken and kill us.

That is certainly the case with COVID-19, which was an obscure and little-known virus just four months ago. In the eyeblink of those few months, globalization has enabled it to spread to every continent (with the possible exception of Antarctica).

Barring a nuclear catastrophe, emerging zoonotic diseases — particularly viruses — are the most pernicious and costly threat to human health and well-being now and in the foreseeable future. The World Health Organization correctly predicted in 2014 that the origins of future pandemics were likely to be zoonotic. The terrifying hemorrhagic fever Ebola originated in a fruit bat or primate in Africa. MERS, a coronavirus, originated in camels in the Middle East. SARS, another coronavirus, may have originated in horseshoe bats in southeast Asia, and the 2002 epidemic may have cost the Chinese government over $20 billion before it was brought under control.

Ironically, the transmission of these lethal viruses to Homo sapiens was almost certainly due to our species’ actions, not theirs. Researchers believe that human cases of viral infections by Ebola and COVID-19 were not caused by chimps or bats or pangolins biting people; rather — in most cases — transmission occurred when people brought, stored, sold, interacted with, killed or consumed animals, usually in so-called “wet” markets.

In such markets, wild creatures are crammed together in squalid and cruel conditions, where animal excrement, blood, organs and humans commingle. Given that many of these creatures carry viruses with exceptionally high mutation rates, it would be difficult to design a more effective means of exposing the human race to potentially lethal disease.

While many of these markets may serve as primary food sources for rural populaces, they also supply trafficked wildlife for the exotic pet trade (reptiles, birds, amphibians, mammals, fish, insects and even arachnids), fashion accessories, trophies and even folk medicine.

A multibillion-dollar industry, global wildlife trade is a mix of legal and illegal activity, the latter more dangerous because clandestine and unregulated; it is also more abusive of the animals and most likely considerably larger than the legal sector. Dr. Daan van Uhm, Assistant Professor of Criminology at Utrecht University in the Netherlands, recently estimated that global wildlife crime may generate over $20 billion annually, making it one of the world’s most lucrative black markets, third only to the drug trade and the arms trade in terms of profit.

I see this firsthand in the Amazon, the region where I work. The legal trade in Amazonia — which includes live turtles and parrots for the pet trade, crocodilian skins for the fashion industry, and jaguar teeth for Chinese “medicine” — exports 14 million animals per year. It is complimented by a huge illegal trade. Overhunting can cause “defaunation” — the depletion of large animals (particularly mammals and birds) that can disrupt a variety of ecological processes, leading to extinctions of both plant and animal species.

With regard to the zoonotic threat, overhunting floods local markets throughout Amazonia as it does in Africa and Asia. According to Dr. Christopher Walzer, Director of Global Health at the Wildlife Conservation Society, Amazonian creatures can be presumed to harbor just as many viruses as African and Asian animals that gave us Ebola, SARS, and COVID-19.

Throughout the tropics, other human-induced factors are exacerbating the problem. Rapid population growth is driving people into these rainforests, where they encounter animals carrying deadly diseases. Construction of hydroelectric dams, roadbuilding, deforestation, mining, cattle ranching and the logging of pristine areas then facilitate pathogen exchange — viruses, bacteria and protozoans jump the animal-to-human barrier. Climate change promises to bring pathogen such as malaria, whose carriers thrive in warmer climes, deep into the temperate zones.

In the short term, diminishing the wildlife trade represents the most effective way of warding off future pandemics. Both the Wildlife Conservation Society and Global Wildlife Conservation have called for a complete ban on the wildlife trade and — in particular — live markets. More environmental organizations are expected to follow their lead.

To their credit, the Chinese recently instituted a ban on the wildlife trade and wildlife markets related to consumption. But they left a gaping loophole: wildlife can still be collected and sold as pets and for medicinal products. Many of these dangerous practices thus persist. A case in point: according to numerous sources, the Wuhan market where Covid-19 originated has reopened.

The moral and practical lesson to be drawn from human-induced environmental threats such as climate change and Covid-19 is simple: there exists a direct correlation between our lack of respect for nature and our own well-being. Not reaching global consensus on this matter is tantamount to willfully enabling the spread of more zoonotic pathogens capable of triggering additional pandemics. In the words of the prescient Lawrence Wright, “we would be naïve and prideful to believe we have escaped the snares of disease that nature is constantly devising.”

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Mark J. Plotkin, Ph.D., serves as President of the Amazon Conservation Team (www.amazonteam.org). His most recent book is The Amazon: What Everyone Needs to Know (Oxford University Press, 2020).