I am not a historian of public health. The facts here come primarily from D.A. Henderson's autobiography Smallpox: The Death of a Disease (2009). I used an LLM to help draft sections of this post, but I’ve edited/rewritten it extensively.
Forty-six years ago today, on May 8, 1980, the World Health Assembly officially declared smallpox eradicated from Earth. It was the first and so far only time humanity has deliberately driven a disease to extinction. The United States alone reportedly recoups its contribution to the program every 26 days in money not spent on vaccination and treatment. Most people treat this as a fixed feature of the world. It is not. If a small number of specific decisions had gone differently, smallpox would probably still be with us today.
I do not mean it would still be killing two million people a year. That is the extreme worst case. But the campaign could easily have stalled for decades, the way the polio and guinea worm campaigns have stalled, with the disease grinding on across endemic pockets, periodically flaring up and being knocked back, never quite finishing. Add modern vaccine misinformation, which barely existed in the 1970s, and you can imagine smallpox eradication still being a "five years away" project today.
Eradication is hard
Before smallpox, four eradication campaigns had failed: hookworm, yellow fever, yaws, and malaria. Smallpox is the only one we have actually finished. Polio eradication launched in 1988 and is still going thirty-eight years later, endemic in Afghanistan and Pakistan. The Guinea Worm Eradication Program began in 1986 and reported 10 human cases worldwide in 2025, but might take even longer because animal reservoirs have emerged as a late obstacle.
Polio has a vaccine that is safe for HIV-positive people, which the smallpox vaccine is not. Guinea worm requires no vaccine and no medicine at all, just clean water and behavior change. And both campaigns have benefited from forty years of accumulated expertise in disease eradication that did not exist when smallpox began.
The policy near-misses
Smallpox eradication was proposed in 1958 by Viktor Zhdanov of the Soviet Union, at the first World Health Assembly the Soviets attended after a nine-year UN absence. Delegates were eager to show solidarity with Moscow’s return to multilateral institutions. The proposal passed by acclamation. The Soviets later provided 1.5 billion doses of vaccine. If the Soviets had returned to the WHO a year earlier or later, or if Zhdanov had proposed something else, or if the political mood had been different, this might not have happened when it did.
The 1959 resolution provided almost no money, and progress was dismal for seven years. In 1966, the Intensified Smallpox Eradication Programme came up for a real vote. The WHO Director-General, Marcolino Candau, was personally opposed and warned that another failed eradication campaign would destroy the organization. The resolution passed by two votes. If three delegates had stayed home, the program would not have launched. Candau's response was to appoint an American to lead it so the blame would land on the United States when it failed.
The campaign
The American was D.A. Henderson, who did not want the job and was, by his own account, "obviously underqualified." He took it anyway. Henderson recounts in his autobiography that his plane was hit by a bird strike during the campaign, nearly crashing, and that the pilot died the same way a month later.
Henderson's nine-person team in Geneva ran the program partly by inventing creative workarounds for institutional dysfunction. When they were officially required to communicate through regional middlemen they did not trust, they would simply CC the actual decision-maker on the official correspondence so the message arrived without going through channels. When a regional WHO office refused to cooperate with the program, the team negotiated to have that region's budget allocation transferred elsewhere until a more cooperative regional director came in. None of this was in any procedure manual. It was made up by the people on the ground because the official channels did not work.
Some of the most important moments in the campaign came down to one person making a call that was not their decision to make. During the Nigerian Civil War, CDC epidemiologist Bill Foege ran out of vaccine in eastern Nigeria and improvised: instead of attempting mass vaccination with insufficient supplies, he vaccinated only close contacts of confirmed cases. The outbreaks stopped. This became surveillance-containment, or ring vaccination, and it became the central strategy that made global eradication possible. The single most important strategic innovation in the campaign came from a missionary doctor making a judgment call in a war zone with insufficient supplies.
The counterfactual
The program was robust to the specific conflicts it encountered. It does not follow that it would have been robust to all possible conflicts and scenarios, and there is one scenario in particular that is worth dwelling on.
The last case was in 1977. The first clinical reports of what would become AIDS appeared in 1981. The smallpox vaccine used during the eradication campaign was a live vaccinia virus that causes a real, replicating infection in the recipient. In immunocompromised people, vaccinia could cause progressive vaccinia, which is a frequently fatal complication. Smallpox vaccination is therefore explicitly contraindicated for people with HIV/AIDS. The CDC, the ACIP, and the WHO all classify HIV infection as a contraindication for the traditional smallpox vaccines.
Now imagine that the 1966 vote had failed and the program had launched five years later. Or that the Bangladesh reinfection had taken longer to control. Any of these could have meant a campaign still fighting the last endemic pockets in the mid-to-late 1980s. You would be running mass vaccination campaigns with a live virus vaccine across sub-Saharan Africa during a continent-wide HIV epidemic, vaccinating people who did not know they were immunocompromised with a vaccine that could kill them, in health systems that were simultaneously being hollowed out by the epidemic. It is genuinely unclear whether eradication would have been achievable under those conditions.
Don't defer to the adults
It is not too far-fetched to imagine a version of the present where smallpox still exists. Not as the global catastrophe it was throghout history, but as one more permanent burden of human existence: outbreaks in fragile states, periodic international scares, vaccine controversies, travel restrictions, emergency campaigns, and millions of people living with scars that nobody under fifty now remembers seeing. The fact that we do not live in that world is one of humanity's greatest achievements. It is also, uncomfortably, a reminder of how much history depends on things that very easily could have gone differently.
When you look at problems like AI safety or biosecurity, it is easy to assume that the decisive choices are being made somewhere else, by people more qualified, more connected, or more important than you. But in 1966, many of the people running global health institutions thought eradication was unrealistic or reckless. Smallpox eradication was not achieved because everyone in charge immediately recognized its importance. It was achieved because enough people, often with limited authority and imperfect information, kept pushing anyway.
A note on corrections
Some of the claims here are interpretive. If you know this history better than I do and I have gotten something wrong, I would genuinely appreciate corrections in the comments.