Viruses that hunt bacteria were treating dysentery before antibiotics existed
In 1917, Félix d'Herelle identified an invisible agent in filtered sewage that destroyed dysentery-causing bacteria, naming it 'bacteriophage' — a virus that infects only bacteria. He used it to treat patients within a year of the discovery. Antibiotics eclipsed phage therapy in the West, but it's now being revived against drug-resistant infections.