The people did not have immunity against the disease
Answer is A apex
The most significant cause of death among Native Americans after the arrival of the Conquistadors was the introduction of infectious diseases, such as smallpox, measles, and influenza. The Native American population had no immunity to these diseases, which led to devastating outbreaks and decimated their populations.
European disease had a devastating effect on the indigenous population of Native Americans. Smallpox, ravaged whole communities and is thought to have been a much more direct cause of the precipitous decline Indian population.
Smallpox
The Spanish conquistadores, after wiping out huge populations with smallpox and other European diseases, for which the Inca had no immunity.
The Aztecs spread diseases like smallpox to wipe out enemy populations.
Perhaps the most devastating result from Spain's conquest of the new world was the radical decline in the population of Native Americas. The Spaniards brought with them a whole host of diseases that the Native American peoples had never been exposed to before. The diseases such as Typhus, Smallpox, Malaria, and Measles wreaked havoc on the Native American populations, killing close to 90% of the population in many areas.
If by people in America you mean "Native Americans", they had not developed the immunities to European diseases, over the centuries that Europeans had developed. Diseases such as measles, diphtheria, even chicken pox, not to mention smallpox were devastating to the American Indian. It is probable that more Native Americans died of European diseases than died in the many battles with the white man.
Aborigines were subjected to more than two diseases with the start of European settlement. Aboriginal populations were decimated by diseases such as smallpox, venereal disease, syphilis, tuberculosis, measles, and influenza.
smallpox
kind of diseases such as smallpox, diarrhea, influenza, etc
Smallpox and other diseases
Diseases, in the form of foreign bacteria. some examples are: smallpox (especially infectious and deadly to those unexposed to it before) and influensa (flu)