To test the hypothesis, he took pus from the cowpox blemishes on a sickly bovine and infected the eight-year-old son of his gardener with it. Times were different, but it's hard to imagine that the gardener was entirely okay with this. Regardless, Jenner waited six weeks and then tried to infect the kid with smallpox. Again, a quick reminder that he was attempting to give a child a deadly disease on purpose. Side note: the history of medicine is full of ethically dubious experiments, many of them performed on the disenfranchised and carried out by, let's be real, rich powerful white dudes, that we choose ignore because society as a whole saw a benefit. It's only some consolation that in this particular case, it all turned out okay: the little boy didn't get sick, because his body had become immune to the smallpox virus. Thus a vaccine was born, albeit a much more pus-filled one than the shots we have today.