Edward Jenner injected the cowpox virus into a patient suffering from smallpox and the patient was cured and never got the disease again.
One peculiar observation by Edward Jenner was that milkmaids who were affected by the non-lethal disease of cowpox were never affected by the lethal disease, smallpox. He injected the cowpox virus into a patient suffering from smallpox. The patient was soon cured and never got the disease again. The viruses were similar in their structure and thus the body was able to develop effective immune cells and molecules against both diseases. This finding lead to the discovery of the smallpox vaccine.