The old hypothesis going back at least to William McNeil's Plagues and People (1976) is that a lot of modern mild diseases were once deadly pathogens.
https://en.wikipedia.org/wiki/Plagues_and_PeoplesFrom the perspective of a virus, there is no evolutionary advantage to killing the host if your goal as a virus is to reproduce and mutate (from an evolutionary perspective, that is the goal of every species). So one strategy is to become less deadly and more contagious.
There are obviously exceptions. One reason for exceptions can be that a virus has a vast non-human reservoir in that reservoir it is non-lethal, but becomes lethal when it spills into humans. Since the human host accounts for a small portion of the virus, there is not a huge evolutionary advantage to becoming non-lethal and there is a pressure to stay lethal, because those viruses that jump to humans do not evolve to lower lethality.
McNeil noted that certain deadly diseases seemed to disappear at the same time that other less-deadly diseases appear and hypothesized that some of the ancient diseases that seem to no longer plague us, actually still do, but in forms that give us a mild fever, a cold, or something like that, rather than killing us outright.
Anyway, I don't know how well that has all held up.
