Rabies Vaccine — Louis Pasteur (1885)

Louis Pasteur's rabies vaccine pioneered post-exposure prophylaxis—proving that vaccination could prevent a fatal disease even after infection occurred, a concept entirely novel in 1885. When Pasteur treated nine-year-old Joseph Meister with an experimental attenuated virus following a severe dog bite, he saved the boy's life and eventually reduced rabies deaths from thousands annually to fewer than 100 globally today.