Edward Jenner should be more of a household name. He eradicated a disease that was killing 50 million people a year in the late 18th century, with a mortality rate of between 10 and 30 percent. He saved an incredible number of lives, possibly in the billions, by creating the world's first vaccine, for which he is credited as saving more lives than any other human in history. By modern standards, however, he may have been viewed as a jerk.
In 1796, smallpox was running rampant in England, especially in crowded cities where the infection spread easily in less than ideal conditions. There was no cure, and the only method developed for dealing with it at the time was to deliberately infect people with a small dose of smallpox pus, known as variolation, in the hope that a mild infection would develop and afterward the patient would develop immunity.
That's a nice way of putting it. The process that physicians had developed – likely as a method of generating more income – involved semi-starvation, bleeding and purging, before deliberate infection (in Jenner's own case, at the age of 8 being thrown into a stable with other infected boys) in a game of Russian roulette for survival and immunity. Though he was haunted by his childhood experience and had adverse health effects that stayed with him throughout his life, Jenner became a physician and administered variolation himself in his practice in rural Berkeley, Gloucestershire.
Jenner, like other scientists at the time, was intrigued by rumors that milkmaids were immune to smallpox. It was thought that dairy farmers were unable to get the much more deadly smallpox after contracting cowpox – an infection in cows. In May 1796 a dairymaid, Sarah Nelmes, came into Jenner's office to ask him about a rash on her hand. He diagnosed her with cowpox, and she confirmed that one of her cows had recently had the disease. Jenner saw this as an opportunity, while Nelmes still had a live infection, to test if it did indeed confer immunity on others.
This is where we reach the part with slightly dodgy ethics, at least by modern standards. He decided to test his hypothesis by injecting the pus of the infected milkmaid into the 8-year-old son of his gardener, young James Phipps. Now, there are a lot of gray areas in medical ethics (one scientist, Stubbins Ffirth, drank fresh black vomit, then smeared it in his eyes to prove that yellow fever isn't contagious, even though it is, which shows how far some scientists will go to avoid injecting their employees with death juice), but cutting your employee's son and rubbing a deadly disease into the wound is in scientific circles a bit of a no-no. Today, ethics panels would worry about potential coercion and informed consent, given the employer-employee relationship.
Young Phipps, predictably, got ill and took 10 days to recover from his cowpox infection. Having successfully not killed a boy, Jenner tried his luck again and inoculated Phipps with actual smallpox this time. It was a huge risk. If he got it wrong, who would mow his lawn? You can't exactly tell the father of the kid you killed, "I think you missed a bit whilst strimming." As was the way with poor families, Phipps also shared a bed with two siblings, potentially risking their health as well.
Thankfully for Jenner's lawn and humanity, it worked. Just to be sure, however, he put smallpox pus into fresh cuts on the boy on 20 more occasions. Luckily for Phipps, each time he showed no sign of infection. He was immune.
Jenner's paper on the subject was actually rejected for publication on first try due to a very small sample size of one (admittedly quite pus-filled) boy. He searched for more volunteers but found few, so gave the inoculation to his own son, as well as a few poor farmworkers and their children. He later conducted a nationwide survey of people who had had cowpox and appeared immune to smallpox, which confirmed his theory.
There had been others who had the same idea and successfully inoculated people against smallpox using cowpox, but it's Jenner who is known as the "father of immunology" for convincing people around the world* to use vaccination rather than the potentially deadly alternative.
*Apart from a few who mocked him, and drew devastating satirical cartoons of people turning into cows.
At the time of Jenner's experiments, nobody raised ethical concerns about the way he treated the boy or other "volunteers" (informed consent wasn't such a big deal back then). The main criticism of his work was along the lines of: "It's an abomination unto the Lord".
“Dr Jenner I must express disquiet. I believe that your project interferes with the natural order," the Rector of Berkeley once told him. "You will know and have mentioned that many religious men raised objections. You refer to only a few of many condemnations.”
There's no doubt that the experiment would have earned him a one-way ticket to cancel town today, but for Jenner's time it wasn't that unusual and pales in comparison to other experiments carried out around and before that period. In 1721, for instance, Charles Maitland was granted permission to test variolation on prisoners, for which they were granted the King's favor and released. The experiment involved infected children sleeping in beds with the prisoners for weeks, which is known in scientific circles today as "Yikes, what the actual hell are you playing at?"
When that was successful, they conducted further trials on orphans. Jenner, meanwhile, ended up purchasing a cottage for the Phipps family. Though the story of the world's first vaccine is questionable by today's standards, as thanks for saving humanity unknowable suffering and contributing towards the only infectious disease humanity has ever successfully eradicated, we can probably refrain from calling him a jerk.