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A provirus is a retrovirus that has integrated itself into the DNA of a host cell. To do this, the RNA of the retrovirus was transcribed into DNA by reverse transcriptase, then inserted into the host genome by an integrase.

A provirus is not active while integrated into a host genome in this way. Instead, it is passively replicated along with the host genome and passed on to the original cell's offspring; all descendants of the infected cell will also bear proviruses in their genomes. Eventually, in response to changes in the host's environmental conditions or health, the provirus will excise itself from the genome again and resume activity as a virus. This results in the destruction of its host as its protein synthesis machinery is hijacked to produce more viruses.

Examples in humans include the herpes viruses.

See also : lysogenic phage -- retroposon