Final answer:
American Indians' lack of immunity to diseases such as smallpox, measles, or diphtheria, brought by European conquerors was due to their lack of previous exposure to these illnesses, which were completely foreign to the American continent before European contact.
Step-by-step explanation:
The American Indians suffered greatly from zoonotic diseases like smallpox, measles, and diphtheria after contact with European conquerors because they had never been exposed to these diseases before. Prior to European contact, the Americas did not have the domesticated animals that were the original sources of these diseases, and as a result, the Native American populations did not develop immunity to them. When Europeans arrived, they unknowingly brought with them pathogens to which the Native Americans had no natural resistance, triggering devastating epidemics.
European settlers, their livestock, and disrupted indigenous practices further exacerbated the vulnerabilities of Native American populations. Livestock destroyed crops, resulting in malnutrition that compromised the Native Americans' immune systems. This, combined with the spread of infectious diseases in densely populated areas, led to tragic mortality rates.
In summary, the correct answer to the question is A) They lacked exposure to such diseases.