[The journey of the vaccine against smallpox: one expedition, two oceans, three continents, and thousands of children].

Spain encouraged, during the Bourbon dynasty, the formation of scientific expeditions, among which was the Royal Philanthropic Vaccine Expedition, an example of biopolitics applied by the state in order to protect health. The expedition went all over the world, using children as a reservoir to transport the vaccine fluid. Francisco Xavier Balmis established a human chain that arm-to-arm materialized the success of the mission. The characteristics and difficulties which children had to pass through and their contribution to the spread of the smallpox vaccine are analyzed.