I'm skeptical of the claim that most viruses stay in our body our entire lifetime, depending on what you mean.
Some viruses do stay in our body our entire lifetime - herpes zoster for instance. It seems to sequester itself deep in the core of the peripheral and perhaps central nervous system in areas that are not readily accessible to the immune system. And it retains the ability to break out and become active at a later date after long periods of dormancy (causing shingles and perhaps some cases of Alzheimer's).
Influenza on the other hand does not remain in the body very long in a healthy human. It is eventually completely eliminated.
If you look at the Baltimore virus classification system, you will observe that viruses are divided into either DNA or RNA types, depending whether their genome consists of DNA or RNA.
DNA viruses are able to enter into latent states, where they live inside our cells indefinitely. So latency is a mechanism by which such DNA viruses can remain in our bodies for our whole lives.
Latency is divided into two types: chromosomal integration, where as you state, the virus inserts its DNA into our own DNA; and episomal latency, where the virus builds a little house (called an episome) within our cells in which to live.
Herpes simplex is an example of a DNA virus, as are all the 9 viruses in the human herpesvirus family (named HHV-1 to HHV-8). Most of us by adulthood are chronically infected with at least around 5 or 6 of these herpes family viruses, with these viruses living in our cells in a latent state. These latent herpesviruses are linked to many chronic diseases. For example, epilepsy may involve a latent HHV-6 infection of brain cells. Chronic latent HHV-6 infection is also found in the uterus of infertile women.
HHV-6 uses chromosomal integration to insert itself into our cells. Most of us catch HHV-6 before the age of 3, so most of us have this virus's genes spliced into our own genes.
DNA virus families include the herpesviruses just mentioned, the adenoviruses (which are found in obese people, and may in part be the cause of the global obesity epidemic), papillomaviruses (which can cause cervical cancer), polyomaviruses, and others.
RNA viruses on the other hand are NOT able to enter into latent states, so in principle should be completely eliminated from the body by the immune response, as they have no "official" mechanism by which to live long term in the body.
The orthomyxoviridae family of viruses is an example of an RNA virus, and influenzavirus is in this family. So in principle, influenzavirus and the hundreds of other RNA viruses which infect humans are not able to form chronic infections, as they lack a latency mechanism.
However, it's not quite this simple: it has been observed that several RNA viruses, including the nasty coxsackievirus B that I caught, are able to undergo mutations in their genome while infecting a human, to become what is known as a defective virus. These defective viruses are then able to live inside human cells as a chronic but low-level viral infection.
Coxsackievirus B and its close relative echovirus (both RNA viruses from picornavirus family) have been demonstrated to live as defective viruses in human tissues in many chronic diseases. This has been proven by taking tissue biopsies from the diseased organs, and testing the tissues for the virus by PCR.
So while it is true that many viruses are fully eliminated by the immune system during the acute infection state, it is also true that many viruses are capable of forming long-term, low-level persistent infections in the body tissues. And studies find that those persistent infections are often linked to a chronic disease.
Edited by Hip, 17 July 2023 - 09:41 PM.
