Final answer:
Indigenous peoples in the Americas had no immunity to European diseases due to isolation and had not been exposed to the animals that originally spawned them, whereas Europeans had developed some resistance. The free-roaming European livestock and other consequences of conquest further weakened Indigenous immune systems.
Step-by-step explanation:
Animal-borne infectious diseases were more deadly for Indigenous peoples in the Americas primarily due to their limited immunity as a result of long-term isolation from parts of the world where such diseases were common. Europeans had developed some resistance to these diseases over the centuries as they lived in closer proximity to the animals that spawned them. The absence of such animals in the Americas meant that Indigenous peoples had never been exposed to diseases like smallpox, influenza, and measles. Furthermore, the introduction of European livestock, which often roamed freely, combined with the consequences of European conquest—including malnutrition due to disrupted agriculture and psychological stress—further weakened the immune systems of the native populations, making the spread and impact of disease much more severe.