Alexander Fleming discovered the antibiotic, penicillin. He found this important substance purely by accident when he observed penicillium mold in a petri dish preventing staphyloccoccus bacteria from growing around the mold.
When he isolated penicillin, Fleming found that the substance killed bacteria but had no effect on animals. Fleming had trouble isolating and producing the substance because it was unstable. Other scientists, such as Ernst Chain and Howard Florey, were able to isolate penicillin and mass produce it for widespread use. The first antibiotic discovered, penicillin kills the dangerous bacteria that causes diseases such as diphtheria, meningitis and strep throat.