phage therapy phage therapy phage therapy, viral phage therapy, or phagotherapy is the therapeutic use of bacteriophages for the treatment of pathogenic bacterial infections. this therapeutic approach emerged at the beginning of the 20th century but was progressively replaced by the use of antibiotics in most parts of the world after the second world war. bacteriophages, known as phages, are a form of virus that attach to bacterial cells and inject their genome into the cell. the bacteria's production of the viral genome interferes with its ability to function, halting the bacterial infection. the bacterial cell causing the infection is unable to reproduce and instead produces additional phages. phages are very selective in the strains of bacteria they are effective against.