Bacteriophage, also known simply as a phage, is a term referring to a virus whose host cell is a bacterium. Bacteriophages have the same wide domain as the bacteria they infect, inhabiting various natural ecosystems. Bacteriophage is a material derived from nature and is harmless to the human body because it already exists within the human body. Bacteriophage can selectively infect specific bacterial strains. This imparts the ability to eliminate antibiotic-resistant bacteria, while reducing the antibiotic dose taken by the patient.