The book mentions population density but quickly discards population density as the primary factor. Instead it focuses on domesticated animals. Eurasians had many domesticated animals and often lived in close quarters with them. The New World had an extreme paucity of domesticable animals and therefore much less exposure to disease vectors and therefore no major disease capable of wiping out populations. (pg 212-213)

Didn't Tenochtitlan have a famously-large population for the era?