Once a global scourge, smallpox devastated millions before Edward Jenner’s vaccine offered hope. But how do you deliver a life-saving cure to the farthest reaches of the world? In 1803, Spain launched the Balmis Expedition, a bold and ethically fraught mission to transport the vaccine across oceans. In this episode, we explore the science of viruses, vaccines, and immunity, while uncovering the surprising and unsettling story of La Real Expedición Filantrópica de la Vacuna.