he coronavirus has devastated the world and killed about 2.3 million people globally. It has infected more than 100 million others, and new variants threaten another surge in cases even as vaccines have begun to roll out.

Yet for all of the devastation Covid-19 has wrought, it’s hard to escape the feeling that it could have been so much worse.

Epidemiologists have estimated that the coronavirus has a “basic reproduction number” between 2 and 3, meaning that when people aren’t taking precautions and practicing social distancing, each infection leads to roughly two to three other people getting infected. (The basic reproduction number is often rendered “R.”) New variants push that R to about 4. Now imagine if it were even more infectious — if the number of people each infection caused were closer to between five and seven. Then there’s the fatality rate. Currently, the coronavirus kills around 0.5 percent of the people it infects. Instead, imagine that it killed 30 percent — and that it would take centuries, instead of months, to develop a vaccine against it.

That’s smallpox.

The horrors of the past year 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.

And smallpox eradication was a big part of it. In the 20th century alone, the disease killed hundreds of millions of people. 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 made the Covid-19 death toll so much higher than it needed to be — 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. As we 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 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 less common Variola minor). It caused fever, then a rash, which over the course of a few days developed into the skin-covering lumps that are the disease’s trademark. The more serious strain, Variola major, killed about 30 percent of people infected with it, with even higher death rates in infants. Death usually occurred within eight to 16 days.

Read the full article about how smallpox was eradicated by Kelsey at Vox.