It’s generally agreed that disease’s introduced by Europeans had a massive effect on the Native Americans. However several hundred years earlier the Vikings made contact with the Natives during their settlement attempts. Why didn’t any diseases transfer between these groups the way they did later?

Hi there, there is a two-part answer to your question. First off, this section of our FAQ may be of interest (as is the section further down about the impact of disease in the Americas on European colonizers.) Second, I'll link here and here our previously written Monday Methods series on why genocide, abetted by disease, is the main reason for the destruction of Indigenous culture in the Americas.