The Biggest killer of all time is SmallPox (Not the Plague). In some socities 90% of the population was killed by smallpox (Smallpox ravished Europe and the people who were left had some sort of immunity to Smallpox) When the Europeans came to the New World, they carried the smallpox virus with them and decimated many native americans.
Smallpox was began on its eliminating by the Cowpox Vaccine developed by Edward Jenner and later by vaccines that contained dead smallpox virii to elimate smallpox, the only disease ever eraticated