Smallpox has been eradicated and the vaccine is no longer necessary, but the infectious disease used to kill millions, and is credited with taking down Pharaoh Ramses V, the Roman Empire, and even the Aztec Empire. It's also credited with inspiring the first vaccination in 1796, when a doctor scratched the infected pus from a cowpox blister into a boy's arm and a few weeks later tried to infect him with smallpox, but found the boy had been inoculated, reports the Baylor University Medical Center Proceedings. By the time of the US Civil War in the early 1860s, vaccination via the less dangerous cowpox virus was common knowledge, and soldiers rightfully feared smallpox. With only one doctor for every 133 soldiers in the North and 324 in the South, many took matters into their own hands when they spotted a cowpox blister, cutting themselves and filling the wound, reports Atlas Obscura.
But cowpox blisters look nearly identical to syphilis, and thus they spread the venereal disease. Their tools of self-inoculation were also primitive and unsanitary; they used rusty nails, clothespins, and pocket knives. The lucky found themselves protected against smallpox, while others picked up syphilis and other infections, and some died or required amputation. As Atlas Obscura concludes: "Even in the most dire circumstances, don’t cut your own arm and fill the wound with your friend’s infected bodily fluids. The results may surprise you." (Melting permafrost could revive smallpox.)