One very interesting book is “Guns Germs and Steel” by Jared Diamond.
He suggests that the old world is oriented east-west, permitting agricultural technologies to migrate easily, while the new world is oriented north-south, so agricutural technologies would have to cross hostile climate bands.
Because of that the old world had superior domestic animals, and superior diseases in their domestic animals to which the native Americans had no resistance. Smallpox (from cattle) gets the press, but probably measles (from Rinderpest in cattle), typhus(perhaps American in origin), plague (from rats) and influenza (from swine) all played a part.
note this article claims the die-off happened years before the Mayflower/pilgrims came.