Sir Alexander Fleming made an accidental discovery that changed medicine. While studying Staphylococcus bacteria under his microscope, he noticed that a blue-green mold that had contaminated one of his culture plates seemed to prevent the bacteria from growing around it. The mold was penicillin, which Fleming realized had antibacterial properties. He spent years after his initial discovery isolating and studying penicillin. His work led to the development of penicillin as the first widely used antibiotic, saving millions of lives by treating bacterial infections.