In 1928, a physician named Alexander Fleming observed that a mold in one of his Petri dishes was killing the bacteria he was trying to grow. This strain of mold led to one of the most significant medical discoveries in history: the antibiotic penicillin. Antibiotics soon became lifesavers. However, even back then, Fleming knew that bacteria could become resistant to penicillin. This video describes how widespread use of antibiotics in medicine, agriculture, and household products can lead to the evolution of microbes that are resistant to multiple antibiotics. Part of the "I Contain Multitudes" series.