Latency is a state in which a virus is inside a host cell but is not replicating and is not making new virus particles. The viral genome is kept in the cell and is passed on when the cell divides. In bacteriophages this is called the lysogenic pathway.Lambda (λ) phage is the classic example. After injecting its DNA into E. coli, the linear phage DNA forms a circle. In the lysogenic pathway it is then inserted into the bacterial chromosome at a specific site, where it is called a prophage. A viral repressor protein switches off the genes needed for lytic replication. Every time the bacterium divides, the prophage is copied along with the bacterial DNA, so all the daughter cells carry it. The host is not harmed and may even gain resistance to infection by similar phages.Latency can be ended by induction. Stress, such as damage to the bacterial DNA by UV light, destroys the repressor. The prophage is cut out of the chromosome and the virus enters the lytic pathway, making new phages and lysing the cell.
Some DNA viruses of humans also become latent. Herpes simplex virus stays latent in sensory neurones, as a circle of DNA in the nucleus. Stress, illness or sunlight can reactivate it and cause cold sores to reappear at the same site. Varicella zoster virus causes chickenpox, stays latent in nerve cells, and can reactivate years later as shingles.While a virus is latent it is hidden from the immune system and from most antiviral drugs.
Some DNA viruses of humans also become latent. Herpes simplex virus stays latent in sensory neurones, as a circle of DNA in the nucleus. Stress, illness or sunlight can reactivate it and cause cold sores to reappear at the same site. Varicella zoster virus causes chickenpox, stays latent in nerve cells, and can reactivate years later as shingles.While a virus is latent it is hidden from the immune system and from most antiviral drugs.