How did people acquire immunity to a disease before the development of vaccines?

Sad fact is that before development of vaccines, not many people acquired immunity to diseases, at least not the very serious ones. The only way for people to acquire the kind of immunity provided by vaccines, was to get infected by a disease accidentally and survive the disease that followed the infection.

As a matter of fact the concept of vaccination occurred to Edward Jenner (1749-1823), by observing that people who acquired cowpox did not catch smallpox. This represented cases of people developing immunity from smallpox when accidentally infected with cowpox.

The fact that one could catch smallpox only once, was well known in times before Jenner. People did try to develop immunity from smallpox by inoculating themselves by smallpox sores. But usually this method proved fatal.