He deduced that cowpox was most likely to transfer immunity from smallpox if administered at the eighth day of the disease.
Cowpox is similar to, but much milder than, the highly contagious and often deadly smallpox disease.
This confirmed his childhood suspicion, and he studied cowpox further, presenting a paper on it to his local medical society.
Cowpox is a poxvirus in the same family as variola.
Vaccinia, cowpox, and monkeypox viruses can infect both humans and other animals in nature.
Immunity to cowpox is gained when the smallpox vaccine is administered.
At some point, the virus in use was no longer cowpox, but vaccinia.
Other vaccine material was not reliably derived from cowpox, but from other skin eruptions of cattle.
