Disease in the New World, by Evan Si

In the first few chapters of the book, disease brought to the New World from the Europeans was discussed. Millions of Native Americans succumbed to a variety of diseases, as can be seen from how the Inca population was decimated from smallpox; over the course of a few years over 60% of the Inca population had died to the disease.

The reason given both in class and in the textbook was a lack of innate protection from new diseases in the Native American people's immune system. But if this is the case, wouldn't Europeans be adversely affected by Native American disease?

The answer is, probably, yes. Among others, the disease syphilis is believed by some scholars to have been introduced to Europe when Columbus returned, having brought it from the New World. 

But as a whole, the impact on Native Americans was much more severe. The reason often given is that Europe simply had more disease vectors, as people lived closer to animals and large mammals not present in the Americas. Moreover, European cities were often less sanitary, and some diseases can only be sustained by a large population that can only be found in Europe. Few places in America harbored this kind of potential for pathogens to spread. 

Nonetheless, these are only partial answers to why disease hurt the Native Americans so severely. There are numerous theories as to the spread of disease between the Old and New Worlds through the Columbian Exchange. Additionally, there are perpetually more questions than answers: why did Native Americans suffer more from the same disease that Europeans were plagued with, even though Native Americans lived in cleaner environments? Was more disease spread to the Americas than vice versa because the Old World was larger than the New World?

So with all this in mind, why did disease act in the way it did?