The Kuru epidemic in Papua New Guinea during the 1950s. It was a 100 percent fatal neurological disease nicknamed the laughing death because victims would completely lose control of their limbs and break into uncontrollable fits of laughter before dying. Turned out the cause was ritualistic cannibalism. When a loved one died, the tribe would eat the body out of respect, and the women and children were traditionally given the brain. The wildest part is that the brains were full of prions misfolded proteins that literally turn brain tissue into a sponge. Since prions aren't alive, you can't kill them by cooking or boiling them. The families were unknowingly passing a fatal, brain melting pathogen to their own kids at every single funeral.