Vaccinations - Small Pox
I recently went down a rabbit hole I never expected. I was reading a bit on Quora, and came across an article about how, 250 years ago, 'a 52 year old Massachusetts doctor turned major general died screaming and blind in a tent on the Richelieu River, and the United States lost its best shot at making Canada the fourteenth state.' Hmm, interesting.

General John Thomas, a country physician from Kingston, Massachusetts died in that tent, of smallpox. His death, and the huge losses the army was suffering due to smallpox, led George Washington to have the entire Continental army inoculated, in secret, 8 months later.

Ok, so, ummm, I didn't know the history of smallpox, that there were inoculations before and during the Revolutionary War, and I didn't know about the difference between inoculations and vaccinations (which were developed in the late 1700s and early 1800s). Wanting to learn more led me further down the rabbit hole.

Inoculation or variolation (from the article on history.com)