Bacteriophage

Natural virus that exclusively infects bacteria, with no action on human cells. Co-discovered in 1917 by French biologist Félix d'Hérelle. A strictly lytic phage attaches to a single species, sometimes a single strain, injects its genetic material, hijacks the bacterial machinery to replicate, then bursts the bacterium (lysis). This host specificity explains why phage therapy does not disrupt the surrounding microbial ecosystem, unlike broad-spectrum antibiotics.