The facts or surmises before Jenner at this date, then,

were--(a) Cowpox produces an eruption extremely like that of
mild smallpox, it is, therefore, probably a form of smallpox
modified by transmission through the cow; (b) And an attack of
cowpox protects from smallpox
The facts or surmises before Jenner at this date, then,
were--(a) Cowpox produces an eruption extremely like that of
mild smallpox, it is, therefore, probably a form of smallpox
modified by transmission through the cow; (b) And an attack of
cowpox protects from smallpox. To test these things
experimentally some one must first be inoculated with cowpox,
and, having recovered from the vaccinia, that same person must,
secondly, be inoculated with the virus of smallpox or be
exposed to the infection, and, thirdly, this person ought not
to take the disease.