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

Before the development of vaccines, people acquired immunity to diseases primarily through natural exposure and infection. When a person was infected with a disease-causing pathogen (such as a virus or bacterium), their immune system would respond to the infection by producing antibodies and specific immune cells that could recognize and destroy the pathogen. If the person survived the infection, their immune system often retained a memory of the pathogen, which meant that it could respond quickly and effectively if exposed to the same pathogen in the future. This immune memory provides what is known as "natural immunity" to the disease.

There are two main types of natural immunity: active and passive immunity.

1. Active immunity occurs when an individual's immune system encounters a pathogen, fights it off, and then retains a memory of the pathogen, providing long-lasting protection.

2. Passive immunity, on the other hand, is a temporary form of immunity that occurs when an individual receives antibodies from another source, such as from their mother through the placenta during pregnancy or from breast milk after birth. Passive immunity can provide immediate protection, but it does not result in long-lasting immunity since the individual's immune system did not produce the antibodies itself.

Before the era of vaccines, surviving an infection was often the only way to develop immunity, and this process could be dangerous or even fatal, depending on the severity of the disease and the health of the individual. In some cases, people engaged in practices like variolation (deliberate exposure to a less deadly form of Smallpox) to induce immunity, but this practice carried significant risks.