In Latin, “cow” is “vacca” and “cowpox” is “vaccinia”. From these roots, the term ‘vaccination’ has come into our usage.
Vaccinia Virus is also the name of a poxvirus (species Vaccinia virus of the genus Orthopoxvirus) that differs from but is closely related to the viruses causing smallpox and cowpox and that includes a strain used in making vaccines against smallpox.
Famously in 1796, Edward Jenner, an English physician realized that milkmaids who had had cowpox did not catch smallpox even during epidemics.
Jenner tried deliberately giving cowpox to people, and found that they were now resistant to smallpox.
This was because the smallpox virus is closely related to the cowpox virus.
It was the first successful vaccine to be developed.