Did you know that in the state of Amazonas right here in Brazil there are a few tribes of native indians that did not had any contact to the civilization?
They did not know that Brazil it's so big. Thereis a govenment foundation taking care of them and making sure that they does not have any contact to civilization tp preservr their roots.
So if we get contact to them they can die. So I think that the study of the diseases will not bring us any clue about the period of time in the case in forum.
Of course for other purposes I believe that we can use diseases to know priods of history. But not in this case.