Thirty years ago, polio infected about 350,000 people per year. But last year, just 21 contracted the disease. That reduction stems from a worldwide increase in vaccination efforts, which has a compounding effect: The lower the number of known carriers, the less chance the disease will spread.

Bill Gates, whose Gates Foundation has heavily funded this work, did some math on the avoided fallout, which he posted to his personal blog in January. Over the last several decades, about 2.5 million kids have been vaccinated, resulting in at least 16 million people without paralysis. One of the biggest innovations in the anti-polio movement, though, is how Gates helped pay for it.

With the help of international aid investment, Nigeria has steadily reduced the prevalence of polio since 2012, when the country accounted for half of all cases globally. Rather than keep the developing country on the hook for the multimillion-dollar tab for that progress, the Gates Foundation offered to repay it, if Nigeria achieved certain benchmarks: Over 80% of the country’s highest-risk areas needed to have more than 80% of their residents covered by vaccines, according to a Gates Foundation spokesperson. As of this year, those conditions have been met.

Read the full article about the Gates Foundation by Ben Paynter at Fast Company.