The smallpox vaccine was exported from Spain to the Americas in the arms of 22 orphans

The vaccine against smallpox was the first vaccine to be developed against a contagious disease. In 1796, the British doctor Edward Jenner demonstrated that an infection with the relatively mild cowpox virus conferred immunity against the deadly smallpox virus. 

In the early 19th century, the vaccine was exported from Spain to the Americas in the arms of 22 Spanish orphan boys. 

Before they left for the colonies, a doctor would give two of them cowpox. After 10 days at sea, the sores on their arms would become ripe, doctors onboard would lance them, and  scratch the fluid into the arms of two more boys and so on.  The ship would arrive in the Americas when the last pair of  orphans still had sores to lance. The doctors could then  hop off the ship and start vaccinating people.

You can read the whole story from The Atlantic here.