So viruses need to be able to spread. If they are too deadly too quick, the infection burns out and the virus may die out with the people who died to quickly to spread it much. So take a hypothetical new virus that is 100% deadly on day three of infection and very contagious. People would die too fast to spread this around the world. So from the virus point of view, it can be deadly, but so deadly so fast to do what it wants to do which is spread itself around. Now viruses don't think of course, and if one existed like the one I made up above it would die out too quickly. So this is sort of where people started getting the incorrect belief that viruses mutate towards less lethality with time. No what happens is there is a selective pressure against a virus that is both very contagious and lethal in a short period of time. They certainly can exist, and perhaps they have in human history but they just died out before spreading too much. Now that is for a very contagious and quickly lethal virus. The virus did not mutate and become less lethal, it just didn't have the right "growth strategy" if you will that worked as far as spreading is concerned. So it may go extinct.

What if you were 100% lethal, but not for many years, say on average 8 years? Well now the lethality is not so much an issue as it allows many years for spread before it kills the person. In that scenario that selective pressure of "burning out" isn't there and it can continue to be lethal and spread around since it has time to spread. HIV is an example of that more or less. Also worth noting Over time it is certainly possible HIV could mutate to a less deadly virus but overall it has not really happened. And there really isn't a selective pressure for it to do so, but things like this can happen anyway over time, but not guaranteed.

Just as an aside, what would have happened with HIV if it happened in say 1900 before we understood viruses like this and really lacked the ability to do anything about it? It would potentially pass through a lot of the population ultimately killing those without some genetic protection that prevented lethality. But we humans have genetic diversity though, and there are some people out there with key mutations out there in one of the HIV receptors who seem resistant to HIV lethality. Over time those that had that key mutation would increasingly become more and more of the population as the others died, and the virus would become less lethal to the population due to the viruses selective pressure put on us humans. Then the virus might be able to infect some but not kill them, or not be able to infect them at all, and as a viral threat would become less and less a threat to human lives. It is thought this may have happened with other viruses throughout our long term evolutionary history. It is possible some viruses we get today that are not deadly to us may have been deadly in the long past but this selective pressure took place and the humans with some genetic resistance are the ones that survived, reproduced and make up more recent human populations. So in this scenario the virus didn't get less deadly, people essentially were selected for who did not succumb.