The video presents the historical development of the cowpox vaccine by Edward Jenner in 1796, which was designed to prevent smallpox. It then describes a controlled experiment conducted by Louis Pasteur to assess the vaccine's efficacy. In the experiment, Pasteur divided 50 volunteers into two groups: one group was vaccinated with Jenner's cowpox vaccine, while the other group remained unvaccinated. Both groups were subsequently exposed to smallpox. The results revealed that 21 out of 25 individuals in the unvaccinated group contracted smallpox, whereas none in the vaccinated group developed the disease. This demonstration of the vaccine's effectiveness contributed to its widespread adoption and played a crucial role in reducing smallpox incidence. The video concludes with a depiction of Jenner and Pasteur shaking hands, symbolizing a significant milestone in the advancement of vaccine science.