Sounds like a new plague type disease, see McNeil "Plagues and People." The high mortality rate leads me to think that this was a new plague introduced into a virgin population.
Very high rates indeed. The article I have in front of me (Discover Magazine) goes to greater lengths to dismiss it as having been introduced by the Europeans.
"These symptoms are not consistent with known European or African diseases present in Mexico during the 16th century."
Yes, but it still can be caused locally. Introduced, so to speak, "intra locally." The agent can be remaining latently in the environment (my hunch) and then being released after proliferating by certain environmental conditions.
If there's a long period of latentcy by whatever the agent is, then, in effect, yes, there is then a "virgin population" present when the agent is brought forth. But, the agent may just very well always be present in the local enviroment, and the second agent of change is environmental within that environment, thus exposing the by-that-time "virgin population" in that same location to the infectious agent.
Because populations lose immunity over time. And, sometimes there's no immunity at all to certain infectious agents, particularly those causeing hemorraghic fevers.
hanta virus is an old plague in the Americas and Asia. It kills off the young, healthy adults leaving behind the oldsters and children.