In the image above, bacteriophages attack a bacterial cell. These tiny viruses, often known simply as phages, infect and replicate within a bacterium.
They are some of the most common and diverse entities in the biosphere, and they are found wherever there are bacteria – which is to say practically everywhere. Tests of seawater have found densities of up to 900 million per milliliter.
Because they prey on bacteria but not on multicellular organisms like humans, they have been used as an alternative to antibiotics, particularly in eastern Europe and the former Soviet Union. As more drug-resistant strains of bacteria emerge, interest in the therapeutic potential of bacteriophages has been revived.