There are quite a few branching subthreads talking about the spread of different diseases, different living conditions leading to different immunity levels, and all sorts of ideas around why it didn't seem to spread deadly illnesses back to Europe as much as Europeans spread deadly ones to the Americas. One I don't see much about is that in the initial exploration, settlement, and colonizing groups the traffic of Europeans was largely one way and screened for serious diseases as best they could before being allowed on a ship.
If Europeans became deathly ill in the Americas, they were probably left in the Americas to die rather than being taken back to Europe. The First Peoples from the Americas were not on average traveling to Europe and staying there for months, years, or lifetimes. They were staying among people in the Americas where they could continue to spread the illnesses. Healthy young soldiers, sailors, and merchants could bring both asymptomatic and presymptomatic cases of illness across an ocean to populations who weren't traveling nearly as much in the opposite direction. When entire colonies of mixed ages, genders, professions, and social roles moved permanently from Europe to the Americas, likewise the trips back to Europe also for former Europeans were far less common and included far fewer than the number of people continuing to interact with others in the Americas.
In short, it was probably easier for mass migrations of Europeans to spread one or more cases of a disease to the Americas where it then spread from more prolonged contact with the population than it was for a European to contract a serious illness in the Americas and take it back to Europe on a military or merchant ship.
As to the Norse and smallpox, the Crusades of the late 11th century and the 12th century were a big part of its spread to most of Europe. There's a very good chance I think there was little risk of a Norse ship spreading it in the early 11th century. As you said, it could be a very different world if they had.
while true that infectious diseases would have been unlikely to make it back to europe, at least in the early colonialism period, surely if there were a mysterious new disease that was affecting colonists it would still be known about, and probably still be around
If Europeans became deathly ill in the Americas, they were probably left in the Americas to die rather than being taken back to Europe. The First Peoples from the Americas were not on average traveling to Europe and staying there for months, years, or lifetimes. They were staying among people in the Americas where they could continue to spread the illnesses. Healthy young soldiers, sailors, and merchants could bring both asymptomatic and presymptomatic cases of illness across an ocean to populations who weren't traveling nearly as much in the opposite direction. When entire colonies of mixed ages, genders, professions, and social roles moved permanently from Europe to the Americas, likewise the trips back to Europe also for former Europeans were far less common and included far fewer than the number of people continuing to interact with others in the Americas.
In short, it was probably easier for mass migrations of Europeans to spread one or more cases of a disease to the Americas where it then spread from more prolonged contact with the population than it was for a European to contract a serious illness in the Americas and take it back to Europe on a military or merchant ship.
As to the Norse and smallpox, the Crusades of the late 11th century and the 12th century were a big part of its spread to most of Europe. There's a very good chance I think there was little risk of a Norse ship spreading it in the early 11th century. As you said, it could be a very different world if they had.