Alexander Fleming was a Scottish scientist born in the 6th of August 1881 who was best known for his discovery of penicillin.
One day while leaving for holidays, Fleming stacked a bunch of his staphylococci cultures in a bench in the corner of his lab.
Over the time he was gone, one of the culutures got infected by a fungus, and the culutures surrounding it destroyed, while the cultures farther away from the infection were normal
Upon further investigation, Fleming discovered the substance. This substance was capable of killing many bacteria types related to scarlet fever, pneumonia, meningitis and diphtheria, as well as gonorrhoea. He named this substance penicillin.
Due to some issues with isolating penicillin and its weaknesses inside the body, it was only after D-Day in 1944 that penicillin started being mass produced. It proved itself effective in treating the wounds of American and British soldiers.