The word clue is a variant of clew, meaning a “ball of thread or yarn,” according to Merriam-Webster.

Maybe we should be referring to equination campaigns today, but when it comes to the sprawling way that language develops, it can be hard to get the horse-slash-cow back in the barn. Later genetic testing revealed that those first vaccines may have actually been using a virus closely related to horsepox, not cowpox. Two centuries and change later, vaccines have eradicated smallpox from the planet, and we continue to take inspiration from Jenner’s coinage when discussing vaccines-even ones that don’t come from cowpox pus. Note the similarity between the word for vaccine and the word for cow in a number of Romance languages today: vacuna and vaca in Spanish, vaccino and vacca in Italian, and vacina and vaca in Portuguese, for example. Jenner, the boy, and the human race at large, the cowpox pus provided a strong degree of smallpox protection, and the concept of a smallpox vaccine was born. Jenner decided to introduce the pus from a woman’s cowpox lesion into a cut he made on an 8-year-old boy’s arm.

The story goes that, just before the turn of the 19th century, a British doctor named Edward Jenner observed that milkmaids who had contracted cowpox, or variolae vaccinae, were much less likely to contract smallpox, which could otherwise devastate entire communities. The word vaccine derives indirectly from the Latin for cow, vacca. Edward Jenner Vaccinating a Child Against Smallpox / Stefano Bianchetti/GettyImages

Ideas a history from fire to freud