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.