Spanish flu didn't disappear, it broke out again in the 1970s and the 2009 flu pandemic was caused by another h1n1 strain of the flu. It died out initially because it had infected basically everyone it could reach (some 30% of the worlds population) and the survivors had developed immunity - as if they'd been vaccinated, and those sorts of viruses burn out fast. And to your point about further mutations, yes statistically having more viral particles out there does mean more chance for mutation to occur (the spanish flu is actually a good example of this, where a second wave of the same flu that had mutated to become deadly was actually responsible for most of the damage, and people that were exposed to the first strain before the deadly mutation were in general immunized against the second deadly strain). THere are concerns of coronavirus mutating as it spreads - there was some talk about having identified different subtypes of the current coronavirus pandemic with different lethalities but I don't remember off the top of my head if that ended up being the case. IN general, viruses will continue to mutate and accumulate genetic changes as they spread, and even though most of these changes will have no impact on infectivity/lethality (gain-of-function changes are rare, loss of function changes lead to a dead end, so the no-change mutations will pile up slowly), you can still use them to track disease spread/understand at what time people got the virus and where they got it from based on the substrain. Here's a NYT article on the coronavirus cases in washington where scientists used just that form of mutation tracking/evolutionary biology to figure out what the most likely explanation for how the virus made it to the state and how long it has been spreading is: https://www.nytimes.com/2020/03/01/health/coronavirus-washin...