The first vaccine was tested on an eight-year-old using pus from a milkmaid's blister

In 1796, Edward Jenner inoculated eight-year-old James Phipps with matter from a cowpox sore on milkmaid Sarah Nelmes' hand, then deliberately exposed him to smallpox. The boy stayed healthy, proving cowpox infection protected against its deadlier relative — the founding experiment of vaccination, named after the Latin for cow, vacca.

— Edward Jenner, An Inquiry into the Causes and Effects of the Variolae Vaccinae — 1798 — public domain