After inoculation, vaccination using the cowpox virus became the primary defense against smallpox.
After infection by the cowpox virus, the body (usually) gains the ability to recognize the similar smallpox virus from its antigens and is able to fight the smallpox disease much more efficiently.

The cowpox virus contains 186 thousand base pairs of DNA, which contains the information for about 187 genes.
This makes cowpox one of the most complicated viruses known.
Some 100 of these genes give instructions for key parts of the human immune system, giving a clue as to why the closely related smallpox is so lethal.
The vaccinia virus now used for smallpox vaccination is sufficiently different from the cowpox virus found in the wild as to be considered a separate virus.
After inoculation, vaccination using the cowpox virus became the primary defense against smallpox. After infection by the cowpox virus, the body (usually) gains the ability to recognize the similar smallpox virus from its antigens and is able to fight the smallpox disease much more efficiently.
The cowpox virus contains 186 thousand base pairs of DNA, which contains the information for about 187 genes. This makes cowpox one of the most complicated viruses known. Some 100 of these genes give instructions for key parts of the human immune system, giving a clue as to why the closely related smallpox is so lethal. The vaccinia virus now used for smallpox vaccination is sufficiently different from the cowpox virus found in the wild as to be considered a separate virus.
Cowpox