d'Herrelle discovered a microbe-eating virus that he called "bacteriophage." He made his discovery independently of the work of Frederick Twort, which was published two years earlier. (See No. 2571). 

D'Hérrelle cited several actual reports of successful treatment of bacterial infections by the injection of bacteriophages in animals and humans. These may be considered early attempts at direct gene transfer in vivo (Wolff & Lederberg p. 11). The advent of antibiotics discouraged further investigation in this direction. Translated into English by George H. Smith as The bacteriophage: Its role in immunity. Baltimore: Williams & Wilkins, 1922. Digital facsimile of the 1921 edition from Google Books at this link, of the English translation at this link.

In this book d'Hérelle reported on the results of quantitative work based on the plaque-count, and dilution methods of assay that he invented. He described a three-step process for the life history of the bacteriophage virus: 1. Attachment to the susceptible bacterium, 2. Multiplication in the cell, 3. Disintegration of the cell to free the progreny virus particles and attachment of the progreny to other susceptible bacteria, if they are present.
English translation as The bacteriophage and its behavior. Baltimore: Williams & Wilkins Co., 1926.