Out of everyone who ever existed, who has done the most good for humanity? It's a difficult question.

In researching for an answer, I came across a list that Esquire had published called "The 75 Best People in the World." The writers suggested that the number one spot should go to… Matt Damon.

This seems unlikely.

A good contender for humanity's greatest achievement is the eradication of smallpox. Smallpox was a horrific disease. In the twentieth century alone smallpox killed more than three hundred million people — more than the total death toll in that time from all wars, all genocides, all terrorist acts and all political famines combined. Around 30 percent of those infected died, sometimes from shock because the pain was so unbearable, and even those who survived were usually left badly disfigured. Yet, in 1977, we eradicated the disease.

If we're looking for the Best Person Ever, we could start by looking at those who helped in this effort. In fact, much of the responsibility of smallpox eradication can be attributed to just one man.

In 1966, a 38-year old Ohio-born doctor named D. A. Henderson became the leader of the World Health Organization's (WHO) Global Smallpox Eradication Campaign. Despite just 10 years of clinical experience, and being 15 years younger than most of the other doctors in the program, Henderson excelled at his job. When he took charge of the campaign, he proposed an ambitious goal: to completely wipe smallpox off the face of the planet within ten years.

Astoundingly, his campaign succeeded, and between 1967 and 1971 the number of smallpox endemic countries plummeted from thirty-one to five. In 1977, the last naturally occurring case of smallpox was diagnosed in Somalia, making it the first disease ever to have been eradicated.

Henderson's success resulted in a string of accolades. He won more than a dozen major awards, including the Public Welfare Medal, the National Medal of Science, and the Presidential Medal of Freedom—the highest civilian award in the United States. He received honorary degrees from 17 different universities, and immediately after 9/11 he became President George W. Bush's lead expert on bioterrorism. He was even knighted by the king of Thailand.

But D. A. Henderson is not who I'm nominating for the Best Person Ever.

By the time Henderson was hired, the political will to eradicate smallpox already existed. There was a job opening and Henderson filled it; he didn't even want the job initially. This isn't to say he didn't rise to the challenge or that he wasn't a hero, but if he had never taken the job, someone else would have done so instead. This person might not have been quite as good as Henderson, but it seems very likely that smallpox would have been eradicated all the same. Henderson was acting as an agent, rather than a principal: he was carrying out other people's intentions, rather than creating the idea himself.

Instead, we should look to a much more unlikely hero: Viktor Zhdanov, a Ukrainian virologist who died in 1987. At the time of this writing, he has a mere four-paragraph Wikipedia page, and there are only a few grainy black-and-white photos of him available online. I'm not aware of any major accolades for his work.

In 1958, Zhdanov was a deputy minister of health for the Soviet Union. In May of that year, at the Eleventh World Health Assembly meeting in Minneapolis, Minnesota, during the Soviet Union's first appearance in the assembly after a nine-year absence, Zhdanov presented a lengthy report with a visionary plan to eradicate smallpox. At the time, no disease had ever before been eradicated. No one knew if it could even be done. And no one expected such a suggestion to come from the Soviet Union; in fact, Zhdanov had had to fight internal pressure from the USSR to convince them of his plans. When he spoke to the assembly of the WHO, he conveyed his message with passion, conviction, and optimism, boldly suggesting that the disease could be eradicated within ten years.

Since smallpox was an exclusively human disease, he argued, it would be easier to eradicate than mosquito-borne infections such as malaria. He pointed his earlier success at eliminating smallpox in the Soviet Union despite its vast territory and poor transportation networks. He referenced Thomas Jefferson's letter to the inventor of the smallpox vaccine, Edward Jenner: "I avail myself of this occasion of rendering you a portion of the tribute of gratitude due to you from the whole human family. Medicine has never before produced any single improvement of such utility…Future nations will know by history only that the loathsome small-pox has existed and by you has been extirpated." On behalf of the USSR, he offered 25 million doses of the vaccine, and logistical support to many poorer countries.

By the force of his arguments, Zhdanov was successful. The WHO abruptly reversed its position, agreeing to form a campaign to completely eradicate the disease. Smallpox is still the only human disease to have ever been eradicated, and attempts to eradicate polio and guinea worm have only had such investment because of our success with smallpox. If it were not for Zhdanov's actions, smallpox might not have been eradicated even today. Zhdanov acted as a principal, not an agent, and due to his efforts there are millions of people alive who would otherwise have died.

The lesson for us is that making a difference requires doing something different.

William is co-founder of 80,000 Hours, which advise peoples on how they can use their time on earth to have as big a social impact as possible— "We find that thinking about what would happen even without your actions is crucial for choosing a career path that really makes a difference."