If you just want my idle speculation, my personal guess is that diseases did exist in the Americas, but 1) we lack the ability to see them in the record and 2)

If you just want my idle speculation, my personal guess is that diseases did exist in the Americas, but 1) we lack the ability to see them in the record and 2) the dynamics of how those diseases originated/spread is significantly different between afroeurasia and the new world. One of the problems with archaeology is that things have to be archeologically visible in order for us to realize they exist. Diseases usually don't leave skeletal evidence and many American cultures were big on cremation besides. It's only under very unique circumstances with certain diseases (e.g. the mummies with tuberculosis mentioned previously) that we can clearly see the evidence to say "hey, these people died in a disease outbreak". In Eurasia we rely heavily on literary sources to supplement this issue, but we have few American sources and they're mostly focused on other things. The nature of continents is also substantially different. The Americas were hugely affected by late quaternary extinctions that destroyed the mammalian diversity, a main factor in zoonotic transmission [1]. The places where that diversity exists post-LGM like the Amazon basin are very remote and their pre-Columbian history is not well studied, let alone the history of disease there. We have little understanding of how they integrated into broader American trade networks either. Eurasia's biodiversity is spread across large areas of the continent, which happen to be crossed by continental trade routes. If plague had originated in Venezuela instead of from rodents in central Asia, we might not even know about historical outbreaks and simply label it a new disease when modern farmers in the highlands caught it. [1] https://doi.org/10.1073/pnas.1919176117 https://doi.org/10.1073/pnas.1919176117