The introduction of penicillin which led to the birth of antibiotic treatments, the development of various vaccinations starting with Jenners smallpox vaccination and the introduction of anaesthesia. These three scientific discoveries have saved millions of lives and completely changed how medicine has progressed.
I’ll just add. Discovery of vaccination, the very first was against small pox. Now of course vaccinations are paramount to preventative medicine.
I am not sure whether you would consider hygene, sterile conditions, anaesthesia etc as important discoveries but they certainly were in their time and of course more recently identifying the human genome.
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