The Vikings colony of Vinland survives long enough to establish lasting trade routes between North America and Greenland (and by extension Europe). Vinland is eventually conquered by a Native American nation but the trade routes survive. This causes all the peoples of the Americas to develop immune systems that can fight off the plagues from Afro-Eurasia just as well as the people then.
When Columbus (or his equivalent depending on the butterflies) show up a few hundred years later Europe has far more limited success in colonising the Americas and Europe remains less significant in world history. The Americas prosper but are just as divided between different countries as Europe, and lots prove open to adopting weapons technology from Europe.
This is inspired by the this Cracked article:
http://www.cracked.com/article_19864_6-ridiculous-lies-you-believe-about-founding-america_p2.html?wa_user1=2&wa_user2=History&wa_user3=article&wa_user4=trending_now
Apparently before the plague wiped out most of the Americas population North America had a population similar in size to Europe.