ANSWER: Around 1796, Edward Jenner, an English Doctor had documented that people who worked near cows were completely immune to the smallpox virus thus semi-creating the first vaccine.
By using Dr. Jenner’s original observation, scientists quickly figured out that people working with cattle were picking up a less dangerous cowpox virus which in turn left them completely immune to the deadly smallpox strain. After several months of testing, an 8-year-old girl was injected with the cowpox virus, confirming Dr. Jenner’s original observation had been correct.
Within 3 years, around 100,000 British residents were vaccinated and saved from that deadly virus. With an 80% child death rate, it revolutionized the scientific world as we know it. By 1979, (WHO) the World Health Organization had announced that the infectious diseases had completely been eradicated. It now only exists in a few different labs around the world.