With the arrival of Columbus and the Spanish it didn't take long until plague had spread across the continent and wiped out millions of Native Americans, so why didn't we see a similar effect when Leif Eriksson arrived five centuries earlier and made contact with the natives? Was there or why wasn't there a European induced plague in America around 1000 AD?
Not to dissuade further comments, but check out the disease in the Americas section of our Frequently Asked Questions for previous answers to why we see less demographic impact on Native American communities from early Norse exploration.