In the 1700s, smallpox was running rampant, 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.
Then Edward Jenner – having heard rumors that milkmaids were immune to smallpox due to infection gained from cowpox – conducted a test when one of his patients came in with the disease. Sarah Nelmes – a milkmaid – came to him with an active cowpox infection, which Jenner could use to deliberately infect some children. He – and we're talking about a whole ethical minefield of its own here – first infected the 8-year-old son of his gardener with cowpox, before infecting him with smallpox.
Several more trials, which would probably be thrown out by an ethics board today, and the world had its first vaccine. But the smallpox pandemic was still far from over.
They were faced with much the same challenges we face today with the COVID-19 pandemic, with most of the globe suffering from the disease and in need of vaccines. It would take until 1980 to eradicate the disease, following a global eradication program. That global co-operation would take over a century to achieve, but in the early days of the new vaccine, there was one attempt to provide vaccines to other countries that became the first international healthcare program in history: The Balmis Expedition.
In 1803, King Charles IV of Spain decided to send free vaccinations to the remote Spanish colonies in America, along with the knowledge and resources necessary for the colonies to begin their own vaccination program. He had been personally affected by smallpox, losing several family members to the disease.
The problem was, cowpox pus could only stay viable for a few days. By the time they got to America, the samples would be useless. The team suggested transporting cows to the colonies, maintaining their infections along the way, but this was dismissed because of how difficult it is to transport them. Then they came up with a better/far more ethically dubious way of transporting the disease, so that it remained alive on arrival.
They would store it in some orphans.
If there was one thing the world wasn't short of at the time, it was orphans, so supply was not an issue. The ship took 22 orphans, aged 3-9, and kept the infection alive by infecting two children, then using the pus from their pustules to infect two further children. When they arrived at their destination, they then paid local families to infect their children, to keep the disease alive and ready to innoculate people against the far more deadly smallpox.
The expedition likely saved countless lives, despite the ethically "is this ok?" method of storing a disease inside some orphans, then sending them on an extraordinarily choppy journey while they were ill.