Why did they invented penicillin?

It wasn't invented on purpose. It was a Scottish guy that invented it and the story goes: he left a window in his lab opened and went on holiday, when he arrived back at the lab he noticed that the bacterium he had growing in his petree dishes had grown all over except in circles around a piece of mold or fungus or something that had blown into the lab through the window.

He extracted the stuff that was in the circle that was clear of bacteria and worked out that the substance must either kill or hinder the growth of bacteria.