WI: Americans gain disease early

Lets say, if romans/carthaginians/vikings/whatever are able to enter into America, spread disease and eventually get the people immune what would be different? I can see the spanish failing to even conquer the Carribean seeing they would be MASSIVELY OUTNUMBERED and disease wouldn't kill basically everyone.
 
1. Native American susceptibility to disease is largely genetic, they have far less variation of genes.
2. Disease is always evolving, if they have trade contact with Romans, trade stop for centuries, then start again at 15th centuries, new disease would wipe them out.
3. There are numerous disease. community that have endemic polio disease is no more immune to TB, mumps, etc.
4. Large size of Eurasia, trading pattern in it, and existence of numerous domestic animals would always means that it had larger reservoir of disease than Americas.
5. Americas vertical orientation, small number of civilized high density people, and lack of trade would means less disease can be maintained as endemic within Americas.
 
Lets say, if romans/carthaginians/vikings/whatever are able to enter into America, spread disease and eventually get the people immune what would be different? I can see the spanish failing to even conquer the Carribean seeing they would be MASSIVELY OUTNUMBERED and disease wouldn't kill basically everyone.
Entire populations can't become immune to a disease. Smallpox and other Old World diseases killed Europeans just as easily as it killed Native Americans.
 
Lets say, if romans/carthaginians/vikings/whatever are able to enter into America, spread disease and eventually get the people immune what would be different? I can see the spanish failing to even conquer the Carribean seeing they would be MASSIVELY OUTNUMBERED and disease wouldn't kill basically everyone.

Hispaniola was conquered before the outbreak of disease.
 
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