Smallpox was one of the worst diseases in human history. It killed an estimated 300 million people or more in the 20th century alone; only tuberculosis and malaria have been more deadly. Its victims were often children, even infants.

It is also, among diseases, humanity’s greatest triumph: the only human disease we have completely eradicated, with zero naturally occurring cases reported worldwide, even in the most remote locations and the poorest countries, for over forty years.

Along the way, it led to fierce religious and political debates, some of the earliest clinical trials and quantitative epidemiology, and the first vaccines.

This is the story of smallpox, and how we killed it.