NavList:

From my point of view, the biggest problem with any of these supposed pre-Columbian sea voyages to the New World, whether from Europe or the Near East or China, is the total lack of immunity to Eur-asian diseases among Native Americans which became tragically obvious shortly after the first Spanish voyages to the Americas. So unless those voyages took place many centuries earlier, when the complex of deadly diseases would have been significantly different (e.g. no "black death" in Europe before about 1347), or unless the voyagers who took those early pre-Columbian voyages were astoundingly disease-free (which is possible with small isolated groups, like the Norse in Newfoundland), plague, smallpox and the rest should have been common in the Americas well before 1500.