Cowpox produced strong enough immunity to smallpox that a vaccine likely derived from it completely wiped smallpox off the planet, but it's very much not the sa

Cowpox produced strong enough immunity to smallpox that a vaccine likely derived from it completely wiped smallpox off the planet, but it's very much not the same disease - cowpox doesn't spread effectively in humans and is much less deadly. There's just really good cross-immunity between the two that made vaccination possible using 18th-century technology and understanding of science, even though their last common ancestor seems to be way back in prehistory.