Twort in 1915 and 'D'Herelle in 1917 discovered:

Explanation

Bacteriophages, the viruses that infect bacteria, were discovered independently by Frederick W. Twort in 1915 and later by Félix d'Herelle in 1917. Twort, a British bacteriologist, observed a mysterious phenomenon where bacterial cultures were being destroyed by an unknown agent, but he was unable to study it further. Two years later, d'Herelle, a French-Canadian microbiologist, clearly demonstrated that this agent was a virus capable of infecting and lysing bacteria. He named these viruses bacteriophages, meaning "bacteria eaters." D'Herelle's detailed experiments and descriptions provided solid evidence for their existence, earning him recognition as the co-discoverer and main pioneer in bacteriophage research.