We need to go back to the deadly infectious disease of smallpox when we talk about vaccines. A method called "variolation" was used as much as few hundred years ago, where people were exposed to material from smallpox pustules in the hope of getting protection from the disease. However, this practice posed risks and people might die from it.
In the 18th century, an English doctor, Edward Jenner, noticed that milkmaids sometimes caught the mild disease of cowpox from their herds and then seemed to be protected against smallpox.
Jenner tried inoculating - or deliberately infecting - an eight-year-old boy with cowpox pustules. After he recovered from the mild disease, Jenner exposed him to smallpox. The boy resisted the infection; Jenner had made him immune to smallpox. The doctor then carried out more experiments and published his findings. His discovery pioneered the development of modern vaccination.
