The Plague Years

Smithsonian

According to the Smithsonian Institution, there have been three great epidemiological transitions in human history. Epidemiology is the study of the distribution of epidemic disease. The first era of human disease began with the acquisition of diseases from domesticated animals. Entire ancient civilizations fell prey to diseases birthed in the barnyard.⁷⁷⁷

The second era came with the Industrial Revolution of the 18th and 19th centuries, resulting in an epidemic of the so-called “diseases of civilization,” such as cancer, heart disease, stroke, and diabetes.⁷⁷⁸ Chronic diseases now account for seven out of ten deaths in the United States⁷⁷⁹ and the majority of deaths worldwide.⁷⁸⁰ Thankfully, these diseases are considered “largely preventable” through changes in diet and lifestyle.⁷⁸¹

We are now entering the third age of human disease, which started around 30 years ago—the emergence (or re-emergence) of zoonotic diseases.⁷⁸² Medical historians describe these last few decades as the age of “the emerging plagues.”⁷⁸³ Never in medical history have so many new diseases appeared in so short a time. The trend is continuing. We may soon be facing, according to the U.S. Institute of Medicine, a “catastrophic storm of microbial threats.”⁷⁸⁴

Most of these new diseases are coming from animals, but animals were domesticated 10,000 years ago. Why now? What is responsible for this recent fury of new and re-emerging zoonotic disease?

Starting in the last quarter of the 20th century, medicine has been examining emerging disease within an increasingly ecological framework. The director of Australia’s National Centre for Epidemiology and Population Health says we shouldn’t be surprised by the recent explosion in zoonotic disease given recent environmental changes. “We need to think ecologically,” he said. “These viruses are trying to evolve.”⁷⁸⁵ Just as plants and animals in the wild try to adapt to new environments to spread their species, viruses also expand to exploit any newly exposed niches. “Show me almost any new infectious disease,” said the executive director of the Consortium of Conservation Medicine, “and I’ll show you an environmental change brought about by humans that either caused or exacerbated it.”⁷⁸⁶ To quote the comic-strip character Pogo, “We have met the enemy and he is us.”⁷⁸⁷

Two centuries ago, Edward Jenner, the founder of modern vaccines, proposed that the “deviation of man from the state in which he was originally placed by nature seems to have proved him a prolific source of diseases.”⁷⁸⁸ This observation dates back to the 2nd century, when Plutarch argued that new classes of diseases followed profound changes in the way we live.⁷⁸⁹ The same can be said for animals. “Something is not right,” reflects Professor Shortridge. “Human population has exploded, we are impinging on the realms of the animals more and more, taking their habitats for ourselves, forcing animals into ever more artificial environments and existences.”⁷⁹⁰ We are changing the way animals live.

According to the World Health Organization’s coordinator for zoonoses control, “The chief risk factor for emerging zoonotic diseases is environmental degradation by humans….”⁷⁹¹ This includes degradation wrought by global climate change, deforestation, and, as described by the WHO, “industrialization and intensification of the animal production sector.”⁷⁹² Along with human culpability, though, comes hope. If changes in human behavior can cause new plagues, changes in human behavior may prevent them in the future.⁷⁹³