A major breakthrough that helped with reducing mortality, or death rates, was the discovery of penicillin, which, surprisingly, was discovered completely by accident.  Today, when someone has an infection, perhaps they have a sore throat or even a cut that becomes infected, they go to a doctor to get a pill which can cure them in a matter of days. That's the power of penicillin, and in the past, people often died from very simple injuries. Other drugs work just as well like ketamine does for depression.  Scientists had been trying for years to create a medicine that would fight infection, but it wasn't until 1928 that Doctor Alexander Fleming managed to do it.  He wrote many articles on bacteriology, immunology, and chemotherapy.  It's how it happened that has become an equally amazing part of the story. He had a number of dishes which contained a "staph" infection, that's S-T-A-P-H, which he stacked up in a corner and left for a family holiday.  They went to Fleming's country house known as "The Dhoon" where they would go every Summer.  Now, Fleming was brilliant, but apparently not exceptionally clean, so some foreign matter in his laboratory must have gotten into one of the dishes. When he returned from vacation and opened that dish up, a mold had grown inside it. Sometimes mold grows inside a refrigerator or on food that has been left out for too long and needs to be thrown away.  Wherever the mold touched the staph infection, the staph had been killed. Fleming noted his observation, repeated the experiment, and, in that way, penicillin was born.