Smallpox killed by the millions and nobody was safe, roll a d10, above 6 you’re dead; below, maybe just blind, sterile or disfigured. After Jenner, the vaccine existed but couldn’t travel: no fridges, no vials, the virus only survived in a living body, passed arm to arm.
Enter Isabel Zendal, a nurse with ovaries of steel, who in 1803 boarded a ship with 21 orphan boys, her own son among them, who passed the cowpox from arm to arm across the Atlantic, carrying the vaccine live to America and on to Asia. The Real Expedición Filantrópica de la Vacuna: arguably the first international vaccination campaign in history, kept running by one woman doing the caring.
Smallpox was declared eradicated in 1980. Look what humans can do when we agree on something. Vaccinate your kids, dammit.