We have some viruses as an approximately stable part of our DNA. These viruses usually stay put within the genetic code. We pass them on to our children and they to their children and so forth. IOW what we call humans are partly viruses. These integrated viruses aren’t usually an issue, although occasionally they will break free and start replicating, usually as a result of an infection form another virus. Needless to say we can’t fight off these viruses so long as they remain solely in the DNA form.
With the majority of viruses we get infected, they replicate a few gazillion times, the immune system fights them off and they die. End of story. The virus is eliminated from the body.
The herpes viruses are the third type. We get infected, they replicate a few gazillion times, the immune system fights them off and they die… mostly. The problem is that some of the viruses hide within the DNA of the host. IOW they become like the integrated viruses, and this is presumably how we came to stable viruses as part of our DNA in the first place. While bound to the host DNA they disable the normal immune system responses that allow it to detect infected cells. As a result, once you get a herpes infection, you have it for life. Every now and then, a few viral strands start to replicate and infect new cells.
So it’s not strictly the case that we always have viral infections and they are kept in check. Most viral infections are eliminated completely in short order. The herpes viruses are problematic because they can’t be eliminated. An infected person can be temporarily asymptomatic, but they remain infected for life.
Not really. The fact is that all viruses are sexually transmitted because sex is a really good way of transmitting viruses. It’s a process designed specifically for transmitting genetic material, of course it transmits viruses. If you have sex with somebody with influenza or rabies, you have a good chance of contracting sexually transmitted rabies or influenza.
What we normally call sexually transmitted viruses are the ones that can’t be readily acquired through other means. Because the average human doesn’t have many sexual partners over a short period of time, sexually transmitted viruses are also very difficult for the immune systems to eliminate. They need to be. A disease spread through sneezing can propagate quite readily even if every single host eliminates it within 7 days, because in 7 days it is guaranteed to be transferred to a new host. A virus that can only be transmitted sexually that can only survive in each host for seven days is going to become extinct almost instantly, because almost no hosts will have sex with a new partner within that window.
We don’t have lots of sexually transmitted viruses because it’s a really niche existence for a virus.
You can be infected with a virus and not express any symptoms, if that is what you are asking. HIV is the most obvious example of that, where most people are asymptomatic for years following infection.
No, it’s relatively rare. That’s why viral vaccines work. Once you contract most viruses they can be eliminated completely by the immune system.
It’s not only common, it’s fairly much universal. If you have herpes then you will occasionally be shedding the virus even while asymptomatic or with only vague symptoms that you would mistake for a mild cold. How often that is depends on the individual. Some people might go years without shedding, others seem to shed nearly constantly despite being asymptomatic. But everyone with herpes is infective at least some of the time despite being asymptomatic.