It is well known that "Spanish" Flu originated in Kansas. It certainly would have spread beyond in the absence of USA military action, but it also certainly would not have killed 100 million people. Exposure rate drives incidence rate, which in turn drives mortality. If only ten people in a location have been exposed and only one of those has become infected, the resources of that location are better able to cope with the epidemic than would be the case if 1M were exposed and 100,000 were infected. Both treatment and isolation are more successful with more resources. This was why Americans were urged to isolate in the initial stages of covid, to prevent a shortage of hospital beds.

A very wrong way to handle an epidemic is to transfer tens of thousands of people to ground zero, expose them for weeks, and then transfer them to various other continents.