Answer:
Alexander Fleming's discovery of penicillin came about by accident.
Fleming was an untidy worker, often leaving his equipment uncleaned. When he went away for a holiday during 1828, he left a clutter of plates growing various bacteria lying about his desk. After he returned, whilst working on an influenza virus he noticed that mould had grown on a staphylococcus culture plate. Not only that, the mould had created a bacteria-free circle around itself. Working on an hypothesis, he experimented further to determine that even a weaker-strength mould culture prevented growth of staphylococci. Thus, Fleming initiated the development and practice of antibiotic therapy for infectious diseases.