There are multiple threads which pose what would happen in the Amerindians obtained diseases that would give Europeans a dose of their own medicine.
What if the Diseases that the Europeans transmitted to Native Americans became more virulent as a result of wiping out millions of Native americans, and came back to Europe in it's virulent form?
What if the Diseases that the Europeans transmitted to Native Americans became more virulent as a result of wiping out millions of Native americans, and came back to Europe in it's virulent form?