teded:

The first smallpox vaccination was tested on James Phipps. He was initially given a small dose of cowpox (blue), which was not lethal to humans. Afterwards, he was given a small dose of smallpox (red) and showed no symptoms of infection. Thus, the smallpox vaccination was developed (and variolation became a thing of the past).

How we conquered the deadly smallpox virus – Simona Zompi

Animation by Augenblick Studios