Hi, just here to make some corrections. As I understand, yes, there are some viruses that reverse transcribe and then integrate into the host DNA, this is a really good example of virus that “hide” from the immune system. They usually activate or make more damage when the infected person goes trough a period of immune system weakness. This kind of viruses are called retroviruses. HPV is not a retrovirus, and it uses a different hiding strategy. This virus can “hide” from the immune system getting kind of dormant in the brain cells. When your immune system is weak, it activates again. Viruses are not really considered living organisms (well, it depends who you ask) but they pretty much follow evolution rules, just that much faster, because of the type of molecule they use as genetic code (RNA virus genomes are much less stable than DNA viruses) and because generally, the polymerase protein they use to make new copies of the genome lacks what is called “proofreading” activity, making them commit more “mistakes” when copying their genome. So when you get virus multiplied, you don’t have millions of copies of the exact same virus, but you got millions of viruses that are really similar, but not exactly the same. Some of this mutations makes some of these new viruses more resistant to certain actions of the immune system, so the original virus may get eliminated, but the mutated virus don’t. This process repeats multiple times, so your immune system constantly recognizes and find new ways to fight the new variants, but the new variants keep appearing all the time. At the end, who wins this battle and the time it takes all this to happen gets you the final result. Maybe someone can contribute more on this issue, or maybe correct something, but that is pretty much what I know. Hope I helped you.

Edit: someone make me notice, HPV does not infect brain cells, I was confusing it with HSV-1.