In 1928, scientist Alexander Fleming returned to his lab and found something unexpected: a colony of mold growing on a Petri dish he'd forgotten to place in his incubator. And around this colony of mold was a zone completely and surprisingly clear of bacteria. What was this mysterious phenomenon? Allison Ramsey and Mary Staicu detail the discovery of penicillin and how it transformed medicine. [Directed by WOW-HOW Studio, narrated by Susan Zimmerman].