how the world’s worst pandemics finally ended

As human civilizations flourished, so did infectious disease. Large numbers of people living in close proximity to each other and to animals, often with poor sanitation and nutrition, provided fertile breeding grounds for disease. And new overseas trading routes spread the novel infections far and wide, creating the first global pandemics.

Here’s how five of the world’s worst pandemics finally ended.

1. Plague of Justinian—No One Left to Die

2. Black Death—The Invention of Quarantine

3. The Great Plague of London—Sealing Up the Sick

4. Smallpox—A European Disease Ravages the New World

5. Cholera—A Victory for Public Health Research

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We know how the COVID-19 pandemic began: Bats near Wuhan, China, hold a mix of coronavirus strains, and sometime last fall one of the strains, opportunistic enough to cross species lines, left its host or hosts and ended up in a person. Then it was on the loose.

What no one knows yet is how the pandemic will end. This coronavirus is unprecedented in the combination of its easy transmissibility, a range of symptoms going from none at all to deadly, and the extent that it has disrupted the world. A highly susceptible population led to near exponential growth in cases. “This is a distinct and very new situation,” says epidemiologist and evolutionary biologist Sarah Cobey of the University of Chicago.

But past pandemics do offer hints of the future. While there is no one historical example to follow, humanity has gone through several large epidemics in the past 100 or so years that eventually stopped ravaging society. The ways they came to a halt offer guidance to a world looking for ways to restore health and some sense of normalcy. Three of those experiences, Cobey and other experts say, suggest that what happens next depends on both the evolution of the pathogen and of the human response to it, both biological and social.

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In this article, we take a look back at some of the other pandemics that humans have endured. Specifically, we investigate cholera, the Black Death, and the Spanish flu, among others. We will note any similarities and take lessons where we can.

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Pandemics end when the virus doesn't have enough susceptible people to infect.

The catastrophic 1918 Spanish flu pandemic is thought to have infected 500 million people worldwide, many of them soldiers living in close quarters fighting in World War I. Once the war ended and people dispersed, the spread slowed as people had less contact. But the flu was ultimately halted in part because those who survived it had immunity and the virus didn't hop as easily as it did at the beginning.

If the virus comes into contact with another person but that person isn't susceptible to the disease, then that chain of transmission is snuffed out. If one person infects two, those people together infect four and so on, and eventually, the virus runs out of susceptible people to infect, said Joshua Epstein, a professor of epidemiology at New York University. "What happens typically is that enough people get the bug that there just aren't enough susceptible people to keep the chain going."

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Even a perfect response won’t end the pandemic. As long as the virus persists somewhere, there’s a chance that one infected traveler will reignite fresh sparks in countries that have already extinguished their fires. This is already happening in China, Singapore, and other Asian countries that briefly seemed to have the virus under control. Under these conditions, there are three possible endgames: one that’s very unlikely, one that’s very dangerous, and one that’s very long.

The first is that every nation manages to simultaneously bring the virus to heel, as with the original SARS in 2003. Given how widespread the coronavirus pandemic is, and how badly many countries are faring, the odds of worldwide synchronous control seem vanishingly small.

The second is that the virus does what past flu pandemics have done: It burns through the world and leaves behind enough immune survivors that it eventually struggles to find viable hosts. This “herd immunity” scenario would be quick, and thus tempting. But it would also come at a terrible cost: SARS-CoV-2 is more transmissible and fatal than the flu, and it would likely leave behind many millions of corpses and a trail of devastated health systems. The United Kingdom initially seemed to consider this herd-immunity strategy, before backtracking when models revealed the dire consequences. The U.S. now seems to be considering it too.

The third scenario is that the world plays a protracted game of whack-a-mole with the virus, stamping out outbreaks here and there until a vaccine can be produced. This is the best option, but also the longest and most complicated.

It depends, for a start, on making a vaccine. If this were a flu pandemic, that would be easier. The world is experienced at making flu vaccines and does so every year. But there are no existing vaccines for coronaviruses—until now, these viruses seemed to cause diseases that were mild or rare—so researchers must start from scratch.

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The Spanish flu pandemic of 1918, the deadliest in history, infected an estimated 500 million people worldwide—about one-third of the planet’s population—and killed an estimated 20 million to 50 million victims, including some 675,000 Americans. The 1918 flu was first observed in Europe, the United States and parts of Asia before swiftly spreading around the world. At the time, there were no effective drugs or vaccines to treat this killer flu strain. Citizens were ordered to wear masks, schools, theaters and businesses were shuttered and bodies piled up in makeshift morgues before the virus ended its deadly global march.

By the summer of 1919, the flu pandemic came to an end, as those that were infected either died or developed immunity.