system
Given the following post, respond as if you're commenting on Reddit:
human
How can a virus reside in a host,but not trigger symptoms?

We all are aware of asymptomatic spread of COVID, but what I'm trying to understand is how a virus can reside in a host, not trigger the normal symptoms , and ultimately still spread the virus.

Is the virus kept under control by the immune system? Is it hidden by the immune system and undetected? Is it hidden in other cells?
gpt
Remember that infections are always an arms race between the host and the pathogen. As evolution occurs, each side tends to develop shinier and newer arms against the other. So for hosts, this is in the form of better immune systems and mucosal defense barriers. For pathogens, this is better means of evading the host immune systems and defense barriers. There are several viruses known to lay dormant inside a host without causing overt symptoms because it's either dormant or the host immune system keeps it in check. One example is shingles, caused by the zoster virus. You get infected with zoster as a kid if you get chickenpox. It lies dormant in your dorsal root ganglia where it's kept in check by your immune system. When you age, your immune system wanes and at some point, it can re-emerge as shingles. That's why we vaccinate older people above a certain age with Shingrix - to boost their immune systems against zoster reactivation. 

Herpes simplex is another example (same family as zoster). Herpes infections don't go away - you can treat outbreaks but the virus is still dormant inside of you when an outbreak isn't occurring. 

And then there's the case of all the "junk" DNA that we all have inside our genomes. One theory is that they're the result of integration by symbiotic viruses. The DNA doesn't do anything that we can tell but remember that the purpose of any living (or close to living, in the case of a virus) thing is to replicate. If junk DNA is just remnants of ancient viral genetic material, then those viruses have done a great job of replication - each time the human genome is reproduced, the virus is also recapitulated.