More than a million Americans have died of Covid-19, while the global death toll stands at over 15 million. It has been a horrifying and largely unnecessary tragedy, one that risks repeating itself as new diseases like bird flu knock on our door.

But for all that the world has lost in the last few years, the history of infectious disease has a grim message: It could have been even worse. That appalling death toll resulted even though the coronavirus kills only about 0.7 percent of the people it infects. Imagine instead that it killed 30 percent — and that it would take centuries, instead of months, to develop a vaccine against it. And imagine that instead of being deadliest in the elderly, it was deadliest for young children.

That’s smallpox.

The horrors of Covid have given us a brief glimpse into what it’s like to live in a world ravaged by infectious disease. It’s easy to take for granted now that very few babies in rich countries die of disease in infancy, that most infectious diseases are treatable, and that there are vaccines available when we need them. But humanity only made the transition into that new world fairly recently.

Smallpox eradication was a major part of that. Over the course of the 20th century, country after country fought it back. The World Health Assembly declared on May 8, 1980, that it was gone for good. Its gradual eradication meant ending the needless suffering and death of millions and millions of people every year.

It’s not minimizing the suffering wrought by the coronavirus pandemic — or forgiving the negligence that has left us still unprepared for possible future pandemics — to take a step back and realize that diseases can be much more contagious, and much deadlier, than this one. And there’s something reassuring about the fact that, at least in the case of smallpox, humanity eventually rose to the challenge.

With luck, aggressive vaccination, and ambitious international coordination, we made the toll of infectious disease lower than at any point in history, and though it won’t be easy, we can do it again. But this year’s anniversary of the eradication of smallpox comes at a moment when the US and other rich countries have unforgivably stepped back from the obligation to aid poor nations, and when vaccine rejection is rising at home. In doing so, we not only forget the lessons of how we ended smallpox, which required international cooperation even between geopolitical foes, but leave ourselves more vulnerable for the next great global health threat.

As we hopefully learn how to address current and future pandemics, it is worth understanding what we learned from the great infectious disease fights of the past.

Smallpox, explained

Smallpox has been around for a very long time. It’s believed that pharaohs died of it in ancient Egypt. It devastated the Americas in the early 1500s after being introduced through contact with Europe. It altered the course of the Revolutionary War, with outbreaks in New England that cost the Continental Army the Battle of Quebec.

Its toll throughout history is hard to measure, but in the 20th century alone it is estimated to have killed between 300 million and 500 million people. “In the contest of Smallpox versus War, War lost,” D.A. Henderson, former director of disease surveillance at the Centers for Disease Control and Prevention, wrote in his 2009 book Smallpox: The Death of a Disease, noting that even the most devastating wars of the 20th century — World War I and World War II — had a combined death toll much smaller than that of smallpox.

Smallpox was spread by a virus (technically, two viruses: Variola major and the significantly........

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