On Wednesday night, we got a crash course in how our bodies recognize harmful invaders and fend them off. Neat, right? Our immune systems are well-trained armies that can spot foreign bacteria and viruses -- and even remember them for later.

Class reminded me of one of my favorite stories of where a word comes from: the history of the word vaccine.

Back in the day, lots of people died from the disfiguring diease smallpox. These days, we don't have to worry about "the speckled monster" anymore. We're smallpox-free in large part thanks to Edward Jenner, who lived in England in the 1700s and kept himself busy studying cuckoos, building hydrogen balloons, and listening to dairymaids' idle chatter. During his surgical apprenticeship -- which he started at age 13 -- Jenner heard a dairymaid say that she would be forever free from the scourge of smallpox.

"I shall never have smallpox for I have had cowpox. I shall never have an ugly pockmarked face," the lucky girl reportedly said.

Never get smallpox, you say? Jenner was intrigued. He remembered her words, and decades later when he met a dairymaid who had cowpox, he took some matter from her oozing pustules and injected it into an eight-year-old boy. After a few months, Jenner tried to infect the boy with smallpox.* The boy didn't get sick. Jenner had created a vaccine for smallpox.

So, what are the origins of the word vaccine? Vacca, the Latin word for cow, in honor of the bovine beasts whose version of the disease helped protect the dairymaid Jenner used in his experiment.

* Unusual as Jenner's methods may seem to us, human experimentation was common in centuries past. Perhaps if Jenner hadn't already been inoculated against smallpox -- using a different method called variolation -- he would have tested his vaccine on himself in the spirit of self-experimentation.