Spreading Disease In The Transoceanic Exchanges

In the Transoceanic Exchanges chapter, what stood out to me the most was the rapid timeline of disease and how fast it wiped out entire populations. Some tribes who had no contact with Europeans and those diseases, experienced nearly 90% population drops, forever changing the landscape and course of history. One aspect of this epidemic I do not quite understand however, is why, given random mutation and other methods of spreading disease, did mosquito-carrying disease not make their presence known to the native Americans earlier. As the book mentions, the Caribbean and tropics were perfect breeding grounds for the Aedes aegypti, which carried yellow fever. Why then, did this disease not emerge from either genetic mutation or from another host-to-human