Bacteriophages are widely distributed and exhibit dramatic manifestations both in liquid cultures and on solid media. Felix d’Herelle’s research had two important directions, focusing on (i) the therapeutic use of the phage in infectious diseases and (ii) the biological nature of the bacteriophage itself. From his first studies of clinical samples from dysentery patients, he observed that phage titers rose just as recovery was taking place. The work that attracted the most attention for phage therapy was d’Herelle’s report of the successful treatment of four cases of bubonic plague with an antiplague phage. Phage therapy for cholera seemed to be established as helpful for the treatment of patients with the disease; phage prophylaxis for cholera, however, was less clear-cut. Bacteriophage therapy for patients in hospitals was compared to tetracycline treatment and to fluid replacement alone as a control. In the 1960s, there was a unification of the two main research traditions in genetics, that is, gene transmission and gene expression. Successful investigations of phages and their interactions with host bacteria in natural settings will require both detailed molecular studies and integrated biological research.