Of course is doesn’t always take millions of years, but it can take millions of years and it is not something you should expect will happen quickly. Even in the case of syphilis it still took 50 years. The evolution of syphilis (to a slower disease course) is exactly what we should be most worried about happening with Ebola as it will then be more effectively spread - just because syphilis is not as quickly lethal as it once was it is not something that you want to catch and leave untreated.