Lysogeny refers to a process whereby a virus that specifically infects a bacterium, a bacteriophage (which means "devourer of bacteria"), achieves the manufacture of copies of its deoxyribonucleic acid (DNA) genetic material by integrating the viral DNA into the DNA of the host bacteria. The inserted viral DNA is then replicated along with the host DNA.

The nature of lysogeny remained unresolved for many years following the discovery of the bacteriophage by Felix d'Hérelle in 1915. The sudden appearance of virus in cultures of bacteria was at first thought to be the result of viral contamination. The acceptance of lysogeny as a real phenomenon came almost 40 years later.

In lysogeny no new virus particles are made. Instead, the virus essentially remains dormant, while ensuring that its genetic material continues to be made. A stress to the bacterium, such as exposure of the bacterium to ultraviolet light, triggers...