My understanding is that at one point in history, syphilis was a really nasty disease, very symptomatic and killed very readily, until it passed through enough of the population to either mediate it or that enough people with some kind of natural immunity survived.
Also, look what small pox did to wipe out indigenous populations of the Native Americans and Hawaiians when it first spread through them because there was no natural immunity to something they had never been exposed to before.
This could be the case with Ebola. All it would take for a pandemic is something new.
That is my magic 40 bullets theory. The human immune system has 40 battle ready antibody squads ready for very specific invaders. The list of those new and old invaders needs to be revised over time and exposures. The oldest get retrained as needed if their enemy of the past has not been seen in a few decades or so. If that number exceeds 40, or is presented with a sudden new enemy with no genetic similarities to an old one, it’s likely any immune response will not take place in time. That’s why we have live weakened virus and dead virus immunizations for prevention of dying from a virus. We introduce it’s genetic code or inject some weakened virus into the host (me and you) so our magic bullet squads have time and resources to program our white cells to immediately jump into action should we encounter the real thing.