Phages, also known as bacteriophages, are viruses that infect and replicate only in bacterial cells. They are the most abundant biological agent on earth and are extremely diverse in size, morphology, and genomic organization. Phages consist of a nucleic acid genome encased in a shell of phage-encoded capsid proteins, which protect the genetic material and mediate its delivery into the next host cell. They are clinically significant for several reasons. For example, many highly pathogenic bacterial toxins are encoded by bacteriophage genomes, such that the host bacterium is only pathogenic when lysogenized by the toxin-encoding phage. Phages have been used since the late 20th century as an alternative to antibiotics in some countries, as they are seen as a possible therapy against multi-drug-resistant strains of many bacteria.

Phages have been of interest to scientists as tools to understand fundamental molecular biology, as vectors of horizontal gene transfer and drivers of bacterial evolution, as sources of diagnostic and genetic tools, and as novel therapeutic agents. They have been isolated from around the world and have been the attention of many genome sequencing projects and much metagenomic sequencing effort. Phages are also present in human biomes, and their influence on the regulation of bacterial populations could have an impact on human health.

In summary, phages are viruses that infect and replicate only in bacterial cells. They are the most abundant biological agent on earth and have been used as an alternative to antibiotics in some countries. Phages have been of interest to scientists as tools to understand fundamental molecular biology, as vectors of horizontal gene transfer and drivers of bacterial evolution, as sources of diagnostic and genetic tools, and as novel therapeutic agents.