Relatively frequently, even the most prosperous of human societies could be practically destroyed in a moment. The main reason for this were epidemics. Illnesses mowed down millions of people and emptied entire cities, and even more terrifyingly: there was no way to ward off infection.

Physicians long understood that patients who had survived an epidemic seldom experienced the same illness a second time. It was as if their bodies “memorised” the illness they experienced and learned how to defend themselves against it. Doctors, using this knowledge, began trying to infect patients with lighter forms of dangerous illnesses in order to pass on these defences.

This was quite simple to do with smallpox, a disease that ravaged the global population for thousands of years. Over the course of this mortally dangerous illness, many liquid-filled pustules appeared across the body of the patient. If the contents of one of these sores were put into a fresh wound on another person’s body, the second individual would contract a lighter form of smallpox and be defended from future infection.