Bacteriophages -- viruses that target and kill bacteria -- were one of the most promising medical treatments of the early 20th century, and were used to treat all sorts of infections, from cholera to staph, and everything in between. But by the 1950s, they had all but died out in the West. This episode tells the story of the humble phage, from its discovery in the waters of the Ganges, love trysts ending in a KGB execution, and to its 21st century resurgence to fight antibiotic resistance.